[zbanks]

The rotary magnetic encoder would work pretty well in this case.

It only gives a position out of 360 degrees (actually in 1.4 degree increments), which isn't ideal, but with code you can keep track when it "crosses over" from 5V to 0V.

As long as its not spinning at >~1000RPM, you should be fine.

Alternatively, you could buy a multi-turn potentiometer, which would give you an absolute position of the shaft. For example, a 10-turn pot would give your position over 3600 degrees. However, they physically cannot turn more than this amount: so plan accordingly. Also, keep in mind that you will lose resolution if you use too many turns. (There are 10-bits of resolution, which means each detectable step is about 0.1% of the total value.)