A couple of bandmates and I were talking about the self-tuning guitar idea many years ago. One fellow, an engineer by trade, had an idea for a device using piezoelectronics instead of little motors.

Piezoelectric crystals, when subjected to an externally applied voltage, change shape. If an onboard electronic tuner were to detect an out of tune string, it would send current to a crystal incorporated into a fine tuning device attached to that string. That crystal would deform, placing (or releasing) pressure upon the string until it was in tune, at which point the current would cease. Of course, the amount of deformation piezo crystals undergo when subjected to voltage is very small, so the idea would probably only be feasible for fine tuning. The instrument would have to be reasonably close to in tune to start with.