casey mentioned that he wants to investigate augmented reality. Having played with the accelerometers on my M200, I'm leary of the challenge of maintaining orientation over many integrations in a handheld or head-mounted device. On a car, though, you can auto-orient using the GPS and a roadmap and there'd be generally more slop for output angles. Finally, I imagine the hardware for painting a display on a windshield would be easier to build yourself than the hardware for painting a display on the inside of a pair of glasses.
This guy's even done all the work.
I have zero competence in hardware, though. Sigh. I was a charter subscriber to Make: maybe in another couple years I'll work my way through their primers...