As someone who has gone through the ringers of stolen computers and case numbers without any success, for a mere $15 a year Hidden (Mac-based theft tracking software) might well be worth it. Just ask Joshua Kaufman:
On March 21, 2011, my MacBook was stolen from my apartment in Oakland, CA. I reported the crime to the police and even told them where it was, but they couldn't help me due to lack of resources. Meanwhile, I'm using the awesome app, Hidden, to capture these photos of this guy who has my MacBook.
He was able to retrieve it thanks to Hidden (A $15/year service that tracks your computer for you), his Tumblr blog, his Twitter account, and the Oakland Police Department, who were able to use the information he collected to bust the guy (Though, somewhat disturbingly, it sounds like they didn't act on the evidence until contacted by Good Morning America after the story blew-up on Twitter).
Go to his site and read the timeline.
Update: If you're looking for a free and open-source equivalent, you may want to look into Prey. Thanks to Benjamin Thomas for the tip.