Apple's Sudden Motion Sensor
Amit Singh discovered how to read it, and wrote a couple of demo apps; Matt Webb promptly used it to control iTunes; and finally Guile, a commenter to AccordionGuy's weblog, suggested the Etch-A-Sketch move: turn the laptop over and shake it to reboot.
So, cool yes. But why not useful? Well: despite the name, most laptops are not used on the lap; too low, and often uncomfortably hot. Most of the time, they're sitting on a desk. Matt suggests a possible use of the sensor for mail reading: "tilt right to delete, left to keep, like you're driving through time through your mail inbox". But with a laptop on your desk, this would first require picking it up. Suddenly it's all more hassle than it's worth.
Maybe there's still potential, if the sensor's sensitive enough, to pick up on directional taps on the laptop's housing. But really, as Matt describes earlier, motion sensing is much more useful for truly handheld devices.
It did get me thinking though about new ways to control iTunes; more on this later.