I have one of those generic flashlights that takes a 6v lantern battery. I also have some 10mm (I think) white LEDs I bought at Digi-key a long time ago. I was going to rig it up with the goal being an emergency light with infinite shelf life that lasts for many days or weeks on a battery. I don't know anything about the LEDS but I pulled 50ma out of thin air as a probable current for long life. I was going to buy a driver board from dealextreme, but I don't know if any go this low of current, and they all seem to be round format when they don't particularly have to be. I've also thought about using a resistor but I'm not sure how wasteful that would end up being.