sonde330, your concern regarding accidental discharges with the light is a very valid point, and one apparently shared by many, because Surefire now provides the option of a "system disable switch", which allows you disable the momentary pressure switch, if you don't want to use it at the moment (or ever). These models, which provide 3 switches (momentary, constant, and momentary disable), can be easily identified by their FGA designations. FA designations indicate a light with momentary and contant but not disable switch. F models are the same, just of an older and discontinued generation. If the forearm in question has no designation at all following the model number, it is an older, discontinued model that only has the momentary switch.
Beware though, the FGA models may or may not cost substantially more than their FA counterparts, depending on the weapon the light is for (I have no idea why this is). The Benelli M1 and Remington 1100\1187 FGAs cost the same as their FA cousins ($325 each, when bought direct from surefire.com). The Mossberg 500\590, Remington 870, and Winchester Defender FGA lights, however, cost MUCH more than their FA counterparts, in most cases, a whopping $157 more!
And yes, the price of Surefire replacement forearms also vary from weapon to weapon. The cheapest current production models are $235 retail, and can be found for quite a bit less if you shop around (definitely available for less than $200, my friend. I'd suggest checking out
http://www.botachtactical.com).
If you own one of the pumps, and don't want to pay that kind of extra money for the disable switch, you still have the option of the adhesive switch blocker, which every light comes with. This handy little metal plate will partially cover the switch, which part it covers is up to you. I think most people install it to the rear of the switch, so they can rest their hand there without fear of accidentally triggering the light.
Keep in mind, not only must you buy the shotgun adaptor for to go with the M3, but unless you have inhumanly long arms (which I don't), you'll want to also buy a remote coil to get a normal grip on the gun.
As for constant vs. momentary lighting, they both have their advantges and disadvantges. You can flash and move with momentary lighting, minimizing your exposure time against an armed adversary (or adversaries). The constant switch is a definite necessity for home defense. It allows you keep the light on and hold the gun with one hand, freeing the other hand for tasks just such as picking up the phone and calling the police, opening doors, etc. The M3 and FA\FGA Surefire forearms all offer momentary and constant lighting, so it doesn't really make a difference.
The Surefire replacement forearm is exactly that, a forearm. If you want to avoid subjecting the light to extended daytime range practice, you can easily remove the lamp MODULE and battery stick, and replace it with a Surefire housing plug (although, as far as I can tell, that housing plug is not necessary to buy, all you need to do is remove the vulnerable components, which are the batteries and lamp assembly (you can keep the module body still on the forearm if you want, to prevent dust\debris\etc. from getting inside the housing. Hell, if you want, you can take off the entire Surefire forearm and reinstall the factory forearm, if that's what suits your fancy. Installing a Surefire forearm is a completely reversible procedure.