Just a quick note to say that I've updated the code to both the GPS-based Lego autopilot and the compass-based Lego autopilot. The only major change is that the motor functions are more effecient and reliable. The old code reset the motor encoder with each turn and this was leading to some pretty serious drift after just a few turns. The new code uses a proportional turn algorithm that is not only more flexible but also eliminates drift.
GPS-based LEGO autopilot code (RobotC)
Compass-based LEGO autopilot code (RobotC)