Just about any motor will generate a voltage if spun; even AC motors without any magnets (induction motors) will produce a voltage if there is any residual magnetism in the armature (and if there isn't any left, just start the motor up on AC, let it run for a minute or so, and there will be some left again). I have an old single-phase AC copier motor with an attached gearbox (powerful beast of a motor) that works well for generating voltage like this.
The biggest hurdle to overcome when using such motors (steppers and other AC motors) is getting useful voltage at a useful frequency; basically, routing and using all of the phase coils of the motor properly to obtain the most efficient output for the power being input into the system (ie, wind or water power, usually). Plenty of people have done experiments on this subject, though, mostly for off-the-grid power systems for cabins and homes. You'll also find discussion in homebrew wind generator systems (such as at otherpower.com).
Something also to keep in mind - in theory you could use such a setup as a speed/motion sensor; you could also (maybe?) have a circuit connected (to the Arduino) that, in between pulses to the motor (ie, PWM), you could measure the voltage/current and do something with that (though it may be simpler just to measure the current as a voltage drop across a resistor with the Arduino).