In modern disk drives, the "flying height" between the platter and the head is very small. This increases the possibility of a disturbed head colliding with the platter. Modern drives support "parking" their heads in a safe position under various circumstances. In particular, heads are automatically parked when a system is powered off or sleeping. AMS adds the ability to park the heads in the event of an accidental drop, strong vibrations, or other abrupt movements.
But I think the most interesting fact is that the sensor can be used as an input device, like mouse and keyboard. I would love to write and play a game in which one would, let's say, need to guide a ball through a maze, and make it reach the center of the maze, by tilting the Powerbook in various directions, in order to win.
Given that tilting the PowerBook in various directions can be mapped to mouse or keyboard input (say, the arrow keys), one could readily use this feature for purposes such as:
* Panning across large maps (consider Google Maps: you slightly tilt the PowerBook backwards to go North, tilt it slightly to the left to go East, and so on)
* Vertical and horizontal scrolling in general, say, in a web browser
* Input for games (for example, a flight simulator)