Accelerometers have come of age. Not so long ago, they cost the earth (or around $30) and output either a voltage or a duty cycle which tied up an ADC channel or a timer.
The LIS302, used in the Apple iPhone, is a much sweeter proposition. It has an SPI/I2C interface allowing you to read off the values directly. There are two configurable interrupt outputs which can be set to alert you when a certain threshold has been reached on the X,Y or Z axes, or when the device was tapped or double-tapped.
It's a Wii controller on a chip and only $10 in ones from Digikey.
