http://www.iamshadowlord.com/2008/12/iturn-iphone-and-ipod-touch-hack.html

“The iPhone or iPod Touch has an accelerometer that detects how the device is oriented. As the devices moves off axis (from straight up and down) the screen rotates. I want to use that feedback to control the position of a motor or servo or cause specific events to happen depending on the device’s position.”