I agree with Benthead, I've wanted a cell phone with a light in it for a long time. There's already that nice fat battery pack to power it, so you'd just add the LED.
In fact you could make a Star Tac battery pack with an LED built into one corner, 3.6 volt direct drive with a little pushbutton on the side to activate it. That would make a dandy accessory to sell in phone stores. You could do something similar for some other phones like the Nokia 5100. Unfortunately, most newer phones have completely internal batteries, so a LED would have to be built into the phone itself, which is harder.