How to interpret the accelerometer readings from iPhone
- (void)accelerometer:(UIAccelerometer*)accelerometer didAccelerate:(UIAcceleration*)acceleration
{
    float g = 9.80665f;
    float x = acceleration.x * g;
    float y = acceleration.y * g;
    float z = (acceleration.z + 1.0f) * g;
    // x, y, and z now hold the absolute acceleration in ms^-2
}