For duty use, the 1911 really needs a dedicated weaponlight. It is an area that I have been diligently researching since the inception of the rail framed 1911s, and I think I can offer some useful insight here. The most commonly overlooked points to consider in picking light rail 1911s are light compatibility and holster availability, so that is where we will begin.

First, let's examine the types of mounting rails:
Picatinny (aka 1913): The milspec rail, most commonly found on the integral light rail 1911s. Though the dimensions are supposed to be standardized, you will find wide variances between manufacturers.
Glock (aka Universal): More or less a Weaver spec rail, it has only one cross slot that is narrower than that of the 1913 rails. This has become a second industry standard for rail dimensions, but is relatively uncommon on current production 1911s.
Dawson: A proprietary add on adapter, it is smaller than the 1913 and Universal rails, but is only compatible with lights fitted with the Dawson adapter.

Next, one must consider the various lights. Given the mind boggling selection of lights on the market these days, this list is not a comprehensive compilation. Instead, it is a list of lights with which we have had extensive practical experience and on which we can provide meaningful feedback.

ITI M3, M3x, M6, M6x: This family of 6 volt incandescent lights feature a fixed rail width, but are available with interchangeable cross bars to accommodate 1913 and Universal cross slots. With bodies molded entirely of polymer, the ITI lights are lightweight and available at competitive prices. Unfortunately, the downside is that these lights have not proven to be as rugged as we'd like to see. The nominal 60 lumen M3 has historically been known to last only about 1000 rounds before failure of some sort, typically a lamp breakage. The primary advantage of the line is their light weight and lower price point. The M3x features a larger bezel and nominal 120 lumen output. The M6 and M6x integrate a nominal 120 lumen white light and a visible laser. It is worthwhile to note that the M6x requires removal from the weapon for battery changes, and the laser does not return to zero upon reinstallation. All ITI lights share a common style of shuttle switch for momentary and constant on illumination. A tape switch for activation with the middle finger of the shooting hand is available, but it removes the shuttle switch. The tape switch is extremely fatiguing to use for long searches, so you may consider staying with the shuttle switch. The brighter, redesigned, and ruggedized "X" series lights are the better choice if you decide to purchase an ITI light.

Surefire X200 A/B, X300: These lights feature a machined aluminum body with a polymer switch plate. The three watt Luxeon LED of the X200A provides a focused narrow beam output of approximately 60 lumens with two DL123 lithium batteries. The 5 watt LED X200B model provides a more diffuse beam and wider light pattern. The X200's have both been superseded by the superior X300 with a 170 lumen output. Newer X300 models are pending for 2012 which increase the light output past 200 lumens.  Adjustable rails and interchangeable locking pieces allow for compatibility with different rail dimensions as well as both Universal and Picatinny rails. The X200/X300 will fit into holsters where the ITI M3 is indicated, but will be loose since it is smaller. Choose the dedicated X200/X300 holster where possible, or plan on using a heat gun to modify for fit. Weapon-specific slimline and DG tape switches are available for the Springfield/Caspian/Kimber rail frame. The Surefire tape switches preserve the shuttle switch, providing a variety of momentary and constant on options.

Surefire X400: This is essentially an X300 with a laser underneath it.  A system bypass and light/laser mode selector rotary switch has been added to the rear of the light body.  The laser is bright and features fairly robust adjustment screws for windage and elevation adjustment. The mounting system is simple and solid. Picatinny rail 1911s with an X400 mounted will fit very loosely in a holster for the M6x/TLR-2, but can easily be tightened with a few minutes of heat gun work.  Holster manufacturers have otherwise been very slow to develop models to accommodate the X400.

Streamlight TLR-1: These lights are reasonably priced and are acceptable choices for duty use. Be aware that users have reported wiring failure related issues with these lights, which Streamlight will address under warranty. They feature machined aluminum bodies, a shuttle switch almost identical to the ITI M3, and a simple screw attachment system that works quite well. They fit into similar holsters as for the ITI M3/M3x, and are typically listed directly in the holster fit lists.

Streamlight TLR-2: This is a light/laser combination similar in execution to the TLR-1. The durability of the wiring is unpredictable to poor, and units may lose their white light or laser rather unexpectedly and within extremely short round counts. These are not recommended for use on the 1911 where the steel frame does not buffer the recoil impulse like a polymer frame pistol.

Surefire Nitrolon series: With a body molded of tough, lightweight synthetic and an aluminum bezel, these incandescent lights are 1913 compatible and also available with a Dawson adapter. With the appropriate lamp assembly, these 6 volt lights can provide 60 or 120 lumens of output. The body of the light features a shuttle switch, similar to the crossbolt safety traditionally seen on shotguns and rifles. It is configured as a constant on/off switch or as a system disable, depending on which version you have. Some versions are also available with a weapon specific tape switch in conjunction with the shuttle switch. The main weakness with this system is that the battery compartment is not waterproof. This line of lights has been long discontinued, but you may still find some floating around and they are a good value.

Surefire Military series: Compatible with the Dawson and MR07 adapters, it features a rugged machined aluminum body and is waterproof. It features the DG tape switch which places a small square pad on the front strap for activation by your middle finger. It is a great light, but is both heavy and expensive. This light has also been long discontinued, but is still acceptable for service use.

Surefire Classic 310R/610R: The original integral light mount. Though discontinued, they are still widely in service. The special slide stop on this unit proved to be problematic, and this light system should be passed over in favor of more modern systems.

Lastly, one must consider the availability of holsters.

Keep in mind that a holster that holds the gun only, like the Safariland 560 adjustable holster (which can accommodate all the light rail guns without lights attached), defeats the whole purpose of an integral weaponlight. The separate light carriers that allow "quick attach" of the light when you need it are forgetting a key issue. That is, when you have had a successful search using the light system and have found someone and need to go hands on, how do you now holster your gun? No one should reasonably be able to tell you that you will be able to safely detach the light from the muzzle end of your gun and reholster two separate items under stress. Something as mundane as negotiating an obstacle or climbing up into an attic will also necessitate some reholstering. Fortunately, new vendors and holster options are coming on line all the time with the appropriate type of holster.

The industry mainstay, the Safariland 6004/6280 series, remains at the forefront of your choices for your light mounted 1911. Their 560 and 561 paddle and belt mounted Custom Fit holsters feature moveable rollers and adjustable tension, and any number of the 1911s without a mounted light should be accommodated by these rigs.

We can't keep up with their list, so we'd suggest that you visit Safariland's "Will Fit Search" website page for up to date model numbers. For reference, here are some of the available Safariland 6004 model numbers for right handed, black holsters.

6004-538-xxx 1911 w/ Dawson Rail or SureFire MR07 with SureFire Military Light
6004-5340-xxx Kimber Warrior/TLE, Springfield Operator, Wilson CQB, Nighthawk GRP with SureFire X200 (should also work with Dawson Rail and X200 Combo)
6004-5621-xxx Same guns above with ITI M3 or M6. Also works with the MR07 Mount and X200.
6004-533-xxx 1911 with SureFire Classic 310R
6004-536-xxx 1911 with SureFire Classic 610R
6004-5322-xxx 1911 with Caspian Bolt-on Rail with ITI M6x

The last three digits designate color and handedness. The first two digits of this series pertain to color:

12 Black
51 OD/Ranger Green
55 Coyote/FDE Brown

The last digit of the series pertains to handedness.

1 Right Hand
2 Left Hand

Thus, a 6004 for a Dawson Railed 1911 with Military Light, Coyote/FDE, in right hand configuration would be 6004-538-551.

Keep in mind that a $60 heat gun can make one of the above holsters work for an unexpected gun/light combination. Any of the "should fit" notations below can become "will fit" with a little heat gun work. The 6004-5322-xxx is rather large, and can be heated and reshaped to accommodate a number of differently configured systems.

Blade Tech, Comp-Tac, and Raven Concealment Systems are making a variety of holsters to accommodate the various gun and gun/light combinations.

And now the guns...

Caspian Recon rail frame: The original rails were Glock/Universal spec, but current frames are Picatinny. The dust cover is reinforced. These will fit into the 6004 for the Operator with M3.

Springfield Operator: This gun was originally available with a full length rail, but is now primarily available in the shortened ¾ length rail version. A wide variety of holsters is available for this gun as it is more or less the industry standard for a railed 1911.  It has the smallest vertical height of the dust cover, and as such holsters that fit the Operator will not typically fit other light rail 1911s.

Wilson CQB/Light Rail: same exterior dimensions as Caspian Recon.

Nighthawk GRP Recon: same exterior dimensions as Caspian Recon.

Sig GSR 1911: Rail dimensions are the same as a Caspian Recon, but the slide is a proprietary profile which eliminates ready holster fit with standard 1911 holsters.

Kimber TLE/RL II: Rail is Picatinny spec with a reinforced dust cover. The rail features a longitudinal cut to allow compatibility with Surefire Nitrolon lights.

Kimber Warrior: Same frame as TLE/RL II.

Dawson Precision rail: We liked this setup a lot and had used it since its inception. The rail is fairly small, and the gun will fit into some standard holsters with minimal modification. With a smaller diameter nylon bushing around the tension screw, the standard 6004 will accept a Dawson rail equipped gun without the light installed. With a Surefire Military Series light, it will fit into the dedicated 6004. With a Dawson X200/X300 adapter, it will fit into the 6004 for the Operator/M3 with only a tension screw adjustment. This setup was somewhat popular for retrofitting an existing gun, but a drawback was that you needed to buy a proprietary adapter which had to be semi-permanently mounted to each light.  The adapters were only ever made for the Surefire Nitrolon, Military Light, and X200/X300.  Dawson Precision has discontinued this product as of late 2011 due to low sales.  Due to the proliferation of dedicated light rail frames which will accommodate unmodified lights, we are no longer recommending or endorsing the use of the Dawson Rail.

Various brands bolt/weld on Picatinny rail: It is a functional solution, but holster options can be problematic since all of the rails will create a vertical dust cover height greater than any of the integral rail frames. These provide similar capabilities as an integral rail, but with some added size and none of the advantages of the Dawson Rail's slim profile and light weight.

Surefire MR07: An add on rail adapter from Surefire that features a Picatinny spec rail with one cross slot. It accommodates a mount-specific Nitrolon series light - the P117C (constant on shuttle, with tape switch), the P117D (system disable shuttle, with tape switch), or the X200/X300 with a modified Universal latch. The adapter itself is machined from an extrusion, and features a set screw to tension it against the trigger guard. You can use an Allen wrench or the rim of a cartridge to tighten the screw. It is held against the front of the frame using a special slide stop and the above set screw. The mount is easy to attach to a standard 1911, and provides a non-invasive weaponlight mounting capability. Installation is quick and simple, requiring only the rim of a cartridge and an extra 60 seconds or so to mount during normal field stripping. Due to the plethora of readily available and superior integral rail frames, we no longer recommend the MR07 adapter system for service use.

Given the mind boggling combinations of 1911s and lights, you can see why the holster manufacturers haven't jumped all over tooling up for every one. Likewise, don't jump on some new factory or custom 1911 and light combination before you've thought out how you will holster it. Be a smart end user and select a complete weapon system to serve your needs.