As an owner of two iBooks and a Mac mini, it has struck me that these are all likely targets for a thief. A while ago I recalled this story about how someone used Timbuktu Pro to recover a stolen iMac. I then realised that because we use Apple's Remote Desktop software at work, then all I need is the current IP address of a computer to be able to observe/control it.
So I wrote a perl script to lookup my computer's IP address and email it to me at regular intervals. I then had the idea to make the script determine the location of the computer based on the IP address. This is not always an exact science, but it can often pinpoint the town where the computer is. Recently I also realised that I could use the UNIX screencapture command to also email myself a screen capture (to potentially see what any thief is viewing/typing). This might be of more use to those who don't own Remote Desktop. Finally, my latest source of inspiration occurred after buying an iSight camera for my Mac mini. I discovered the great (and free) isightcapture utility, which now allows me to email pictures taken by the iSight camera.
So I now have my computers send me regular emails throughout the day which get automatically filtered by a rule in my Mail application. Because each of these three different functions (IP address, screen capture, and isight capture) require different tweaks to the system, I wrote three different perl scripts so people can pick and choose what functionality they want. These three scripts can be downloaded as a tarball, or downloaded separately from my blog, which has full and detailed instructions on how to install each script (some require some tweaks to the system).
The main limitations of these scripts are that a) the stolen computers need to be online long enough for the scripts to run, and b) the thief doesn't just wipe your hard drive. I try to counter the second option by not using a password on the screensaver, and setting the computers to automatically log into a somewhat-restricted guest account following any reboot. Hopefully this would encourage someone to use the computer just long enough for you to get some helpful information emailed to you.