Thought you'd have fun with this one:

these programs use the built in motion sensor that was designed to protect the hard drive, to detect movement, and turns it into a motion sensor alarm for your machine!

http://orbicule.com/theftsensor/

or

http://www.slappingturtle.com/home/

How cool!

I've had orbicule installed for a few months and it seems like a neat idea. I use multi alarm for the motion sensing and a mechanical lock for the library. I wear suspenders with a belt too...

Great idea.....I would love to see the face of someone actually trying to steal one and then have the alarm go

While the idea of an alarm is a novel idea and sounds like it would be fun to play with, the best plan is to not leave your notebook unattended.

Laptop thefts are becoming more and more prevalent today as more people are using laptops out and about in coffee shops, libraries and other public places. Thieves tend to work in teams. One person distracts you while the other makes off with your notebook. In those situations, the alarm would be nice. However, I still wouldn't rely on it totally.

One recent scam was the thief would purposely spill coffee on you or near your computer. When you get up to go get something to wipe off with, the thief makes off with your computer. Even if the alarm sounded, you're probably a good 30 feet away at that point, and he/she is running out the door. If they had a car waiting/running, they're in the passenger seat and gone before you could catch them.

Best defense, don't leave your laptop unguarded.