I made use of the hax0r expansion port (R4 and the two holes next to the speaker) to add a dimmer to my Ice Tube Clock. A modified version of the Sept 30th firmware is available here
.... use at your own risk etc....
The dimmer's operation is pretty simple. If the room is pretty dark, the display is at minimum brightness. If the room is light, the display is at the brightness set by the user. There is a menu option to turn the dimmer on and off. For extra credit I could add some sort of sensitivity option to the menu or but I figured I would start simple and see how well it works. If you need to adjust the sensitivity, there is a constant DIMMER_THRESHOLD in iv.h. The dimmer checks the room's brightness every second. It is turned off while the clock is running on battery backup. The dimmer is disabled while in the set brightness menu.
The hardware is just a 10K resistor in the R4 position and a CdS photocell in the sensor position. My photocell's resistence varied between ~0 Ohms in sunlight and around 11K in the dark.