Well, in principal, using the camera will only be able to get you relative motion, not tilt. Unfortunately, I believe doing that would be quite CPU intensive (it's kinda like encoding a movie, since most codecs do exactly that to only store changes of the image between frames). Assuming it's not too CPU intensive, it will also be subject to drift, since it cannot reset to "up" like a gravity-based sensor does. So, yes, in principal, there are algorithms to do this. Practical? Unlikely. Although I'd be happy to be proven wrong 8-).

Martin