I discovered this on my Hack A Day RSS feed…
[Johny Lee] sent in his(pdf) awesome projector calibration project. By adding embedding some optical fiber that feed into a set of USB connected light sensors, his groups software can determine the exact pixel position of each sensor. Once the positions are determined, the projected image can be dynamically adjusted to fix the screen. The technique can be used to stitch together multiple projectors, and even calibrate an image to project onto a three dimensional model.