I had an idea and wanted to run it by people to see if anyone's looked at it before or is interested in pursuing it. There are games for other devices (and now for the N800, see http://www.harbaum.org/till/tiltstick/index.shtml) that use an internal or external accelerometer as a joystick-type input device. I was thinking about whether it would be possible to use the built-in camera as a motion-sensing input device. I know there's the Linux package called motion that can detect and track movement of a single object in the field of view--what if you tracked the movement of the entire field of view and interpreted a movement as a rotation? It probably wouldn't be very precise compared to a real accelerometer, but imagine the coolness factor of being able to just pop the camera out and roll a ball around the screen.

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