[aptalca]

Thanks, i was not aware boblight would work on webcam input. That sounds like a possible solution for me, though I was hoping for something that could be self-contained on the arduino board with just an optical sensor I could aim at the tv.

Arduino board only does the last step of turning the lights on/off and setting the color. The screen capture and the average color calculations at several areas are all done by boblight or whatever software you are running on the computer. That software then feeds that final data to the arduino, which just controls the lights. A better solution is through a tv/video capture card as LastMile suggested.

I've often wondered if it would be possible to use the composite Monitor Out on my TV and some sort of TV card to drive my Atmolight so all my AV devices got pretty colours, not just XBMC. But wondering is about as far as I got ...

It should work on whatever is displayed full screen on the computer as it relies on (full screen) screen captures. You could definitely set up a video capture card, all you have to do is play the feed fullscreen while running boblight.