Quote Originally Posted by tom4bren
LEDs can dim - a little.
No, they can dim a lot.

Here is just one of many graphs of the output of an (unregulated) LED flashlight brightness versus time starting with a fresh set of batteries:



They pretty much all look like that - 50-75% dimming.

Here's a ton of discussion on PWM versus current-control for LED lighting.
The general consensus seems to be that current-control is preferred.
http://www.candlepowerforums.com/vb/...d.php?t=152185