This Youtube video shows a guy’s Joule Thief that has its brightness controlled by a CdS photocell. The ambient light changes the photocell’s resistance, which adjusts the JT’s brightness. I haven’t yet seen the schematic but he promised another video with more will be posted soon.
I left a comment with a link to my blog that shows hot to use a LED as a sensor. Someone mentioned that there was a device that used a microcontroller to drive an array of LEDs, and during the time the LEDs were off, the microcontroller changed to sensing the current from the LEDs caused by ambient light. That’s pretty cool, because you could use them to make an interactive display that senses shadows from your hand, for instance, and changes the LEDs accordingly.