I just heard this tip on the Feb 5th episode of Inside Mac Radio. 2600 - The Hacker Quarterly had an article on how to secure your PowerBook. He includes a php script that the will track the ip addresses that connect to it, and send an email when a new one is seen. Then a cron job is added to the PowerBook that will connect to that php script.
There also is a version of the script that will cause the PowerBook to run a command when it encounters a new ip address. (So theoretically, you could have the PowerBook wipe the drive or shutdown when it is stolen.)
One thing that I may add to this is sending the MAC address of the PowerBook. If you were to to this, you could track multiple laptops with this script.