I came across this article in PC World about an open source app and service that can track lost or stolen laptops for free. It's called Adeona. The Mac version also allows for the ability to take a picture using the built-in i-sight. With the number of laptops being stolen nowadays (I myself being once a victim), maybe this can be of help in the event of a lost or stolen Mac. I haven't had the time to try it yet though.