http://blogs.techrepublic.com.com/itdojo/?p=1525&tag=nl.e101 shows you in a web video (transcript also available) how to configure a free program, Prey, to holler at you when your computer is stolen, then used on the web. As a free service, which works for Windows, Linux and Mac, I can't see how anyone would not want to use Prey or something like it. Now, Lojack for Laptops is a more robust service and would survive a wipe-and-reload (unlike Prey), but, well, it isn't free.
Prey needs either a service for you to alert the laptop is gone, or it can watch a web page; when that web page is missing, Prey assumes the laptop is stolen, and starts e-mailing to you. Being me, well, I prefer using both, and find google.com/sites to be a very good place to create a web page for each computer you want Prey to watch.
Firefound is another free tracker for laptops gone astray, but relies on Mister Perpetrator using Firefox; if he uses Chrome, Safari or IE, Firefound is silent.
And, a lagniappe: Prey now works with Android phones, too!