Hitachi unveiled what the call the world's smallest Triaxial Acceleration Sensor.

Measuring 2.5x2.5x1mm and weighing just 14mg, this component is really small.
The piezoresistive sensor uses the displacement caused by the acceleration of a weight based on MEMS (Micro-Electro-Mechanical Systems) technology is detected as a deflection in the silicon beam supporting the weight. The deflection is measured based on the variation of resistance in a piezoresistor formed in the Si beam. Hitachi Metals will start shipping samples in May 2008.
More details on Tech-On.