The Sudden Motion Sensor (SMS) on the PowerBook is used to turn off the hard drive in the case of sudden acceleratins (like those that occur when you drop the computer, for instance). The ThinkPads have a similar system. The device is almost certainly using a microelectromechanical system (MEMS) sensor, probably one of the ADXL series from Analog Devices -- the same types of sensors that are probably used to control the air bags in your car.
It's possible to read the accelerometers at a user level, too. In general, this
seems to be of dubious utility, unless you want to tilt your computer around as
a way to control something. But I think it's utterly cool that I can download a
little applet that will, in real time, show the orientation of my laptop based on
the accelerometer readings. This demo is going into my
where are MEMS used?
slide for my talk on Monday, and probably for several other talks after that.