A guy I work with bought one of these for his car: http://gps-snitch.com/marketing/moreAbout.html
A couple of days later, his car was stolen. He was able to monitor its whereabouts in real time. When he contacted the police, they looked into who the thief was, and used the GPS tracking info to bust the guy on a whole bunch of different charges, including the car theft.
Does anyone know if they make something like this for bikes? Maybe something you can drop into your seat tube?