MIT's Technology Review has a piece about the latest line of Gibson guitars that feature self-tuning mechanisms inside that can adjust string tuning on the fly.
The system begins with an additional set of pickups mounted underneath the strings that are used specifically for the tuning process. But unlike conventional pickups for electric guitars, which are magnetic, Adams uses piezoelectric pickups. These pickups are made from a material that creates an electric charge when stressed or pressured, such as by the sound waves coming from the guitar's strings.
As the strings are played, the Powertune processor compares their actual frequencies with the desired notes and sends instructions--tighten the string this much, loosen the string by that much--to tuning pegs equipped with strong, tiny servo motors mounted on the back of the guitar's head. Because onstage interference could potentially degrade a wireless signal, the system uses the strings themselves to send the signal.