We were gonna do a spoof of the ipod dancer thing, using a difference matte. In Final Cut (express will do this - Photoshop will do this frame by frame if you export from imovie as an image sequence). Using the diffference composite mode with an arbitrarily plain background clean plate, one should be able to make a difference mask that can be applied to either mask or isolate the forground element. This would work with poorly lit green screens as well so long as you have a clean plate of just the poorly lit screen (and few shadows hit it). Generally you would want a background that is in contrast to the subject you will be adding to the foreground, but you should be able to get something out of just about any background. For our project, we were going to use a plain white wall as a background to be masked out and replaced with a single color background. This would then blink back to regular color when the music stopped.