I built a mobile device for mobile tracking. This device mainly consists out of a GSM mobile phone with a GPS module.
I use this device to send twitter messages with my actual position. This feed can then be used with twibble
to get a track displayed in Google Maps.
The module is programmed in Python. Source and schematics are available.
If that sounds cool, then have a look at tinkerlog:
* Using twitter and twibble for mobile tracking
* DIY mobile tracker