SpyBike GPS Tracker

You can keep track of the bike's location via Integrated Trackers' website, and then relay its location to the police. If you forgot to arm the unit before your bike was stolen, you can remotely activate it by sending an SMS message. It checks its messages automatically every six hours, so it will begin tracking when it gets your message.

http://www.gizmag.com/spybike-gps-tracker/22999/

Fits inside the steerer and is supposedly difficult to remove.