I've finally got around to posting a video of my take on the classic NXT Segway:
My version uses a combination of an accelerometer (homemade), and a HiTechnic gyro to sense the angle/movement of the robot. The values from these sensors are fused using a Kalman filter and the result is used to drive a PID motor controller. The Robot is pretty stable and will stand around longer than I can be bothered to wait! It also responds well to being pushed around. I've experimented with making it move backwards/forwards and turn, but that work is not complete yet....