That is really nice! Where can we find more details?
It's an arduino mega1280 powered by a 7.2V battery pack I made. There's also a 12V battery to power the motors.
The drive is an MD25 motor controller and EMG30 motors. I made a breakout board for the 7.2V power connection and an I2C bus with some extra headers so I can add another processor later. The MD25 is I2C controlled.
I used PID for the motor control (still tuning ...) and an IMU 5DOF accelerometer and gyro for detecting the angle using 2 accelerometer axes, the gyro rate and a kalman filter.
Not sure the motors were a good choice, lot of backlash. Also, I originally had the heavy battery at the top which gives me more time to react to changes as it tilts slower but it also requires more torque to correct it and the motors just weren't up to it. It's quite a heavy RC car battery.
I got a lot of info on here: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1284738418/0