The New Scientist has a write-up on an EU-funded prototype system called Tai-Chi that can turn ordinary surfaces into a touch-pad input device just by attaching a tiny piezoelectric sensor (i.e. microphone) to the surface. In one configuration, the system figures out where you’re touching / tapping by listening to how vibrations are distorted by the object and then either comparing to a database of vibration “fingerprints.” The method requires calibration to create the database, but they’re claiming accuracy to within a few millimeters.