… iAlertU, but the video says it best…
Direct link, because the embedded video is being flakey.
This is not a serious security device, and never will be for as long as the thief is still able to hold down the power button for a few seconds. But it is a nice deterrent, and there is a real market for something between something like the Kensington locks (that in my experience often get bought and rarely get used) and leaving a completely unsecured laptop.
I just finished trying this out and it worked, it is Intel only (shock!) at the moment but apparently PPC PowerBook and iBook versions are in the works.