GPS Tracking for Stolen Bikes

silver you can already do something similar using mobile phones. all you need is a small receiver (a cheap mobile phone) and use a service like this http://www.traceamobile.com/ that essentially connects to the mobile phone being tracked and triangulates its location. i've been looking into developing a bike specific one - and i think i can get it down £20, minus the service.
I thought GSM triangulation was only accurate to about a 20m² :(