There is a new mod out for doom 3.
This attaches the flashlight onto your pistol.
Get it at fileplanet
The Pistol Flashlight Mod 0.1 by John 'Cryect' Rittenhouse [email]firstname.lastname@example.org[/email]
Alright, people have been complaining about not being able to use the flashlight with weapons so I decide for a first mod I would tinker around with that. I've just modified the pistol to have a flashlight and unfortunately there is two issues at this moment, no muzzleflash and no way to disable it (btw in multiplayer reload while using the flashlight to switch it off/on). Its kinda interesting though being able to see the guys as you are shooting them for a change.
To install this mod copy pak005.pk4 to your base directory where your doom installation is (you will see other pak00?.pk4 files as well there).