Actually, there is already a solution made by HiTechnic. They have their angle sensor, but it's the same size as a LEGO sensor, so, it's kind of hard to put it anywhere. Well, Mindsensors just smashed that problem into pieces! Their solution is about 1 stud thick and about the size of an XL-Motor. Introducing, the Mindsensors GlideWheel!
"This Device will have several models -
One model will allow you to connect PF Motors to NXT Motor port. It will have encoder support to control them precisely.
Second model will be similar, for RCX motors.
Third model will be a digital angle sensor, to measure angle with 1 degree precision."
:-O. Great job Mindsensors! Can't wait to see it! :-)