A lot of you already know about this, but I wanted to get it out here just as a convenient place to find the link again. iAlertU is on the slappingturtle.com web site.
iAlertU is a great example of synergising the parts of the Mac. It uses the gyroscope (for parking the hard drive) and any key/mouse motion to trigger it. When you run iAlertU, it appears on the menu bar. Using the remote that comes with the machine, you hold down the MENU key, and you'll get the familiar car alarm "chirp-chirp". The machine is now armed (you'll also notice that the camera turns on). If anyone (like Jay) touches it, it sounds a car alarm, blinks the screen, and snaps a picture of the perp. Closing the lid on the machine silences it, but as soon as it is reopened, the alarm continues.
To turn it off or disarm it, use the remote and hold the MENU key down again. You'll get the single car alarm "chirp", and you're back in business.
Whenever I wander away from my machine around my hot-dogging prone co-workers, or at a conference, I arm my machine now so that it causes a ruckus when disturbed.