I was reading an issue of mac life today and i can accross this great program for macs. Its called iAlertU. Its free so even cooler. you set up your preferences and arm it by typing a password. Once its armed (depending on your preferences) the scrren will go black with there logo (i think a bad idea) or remain the same (good idea). if the keyboard is touched the mouse is moved or devices are put in or taken out. an alarm goes off at full volume. the isight also turns on and starts taking pics and emailing them to you. Sweet program some flaws.

If you are on a network with a cable the theif could take out the cable first. and alarm would go off but couldn't email the pictures.

take a pair of headphones and cut the buds off. but em ing the headphones jack and no sound at all. (maybe use the motherboard sound card instead?)

all this in combonation with holding the power button allows a thief to steal the computers. not only will this pc restart free of noise but the program is completly deactivated.

and on imacs you can hit f10 (the mute button)

good idea but needs work.

but then again what thief comes this prepared. if you are so desperate for money you have to steal... you probably dont own a mac.

There's also Undercover if you're looking for an alternative. It's not free but sounds better than iAlertU in my opinion.