[Tom Bottiglieri]

You dont absolutely need encoders or some other type of feedback on the driven wheels to create a balancing effect. The accelerometer should give you sufficient information needed to start the balance, but the most you will ever be able to get is a platform that rocks back and forth. This will be because, based on the information you've given, you would take the given rotation angle, then use that to drive the motors at a certain speed in a certain direction. The precision on that sensor wont give you enough info to create an exact stopping point, and the motors will overshoot their target. You could of course add a larger deadband, but would only increase balancing problems as the platform levels out in the deadband while off balance, and due to sensor "drift"

As to previous posts: I'm not exactly sure what a yaaw rate sensor would do for you in the aspect of balancing. Maybe to see if the motors are unbalanced while driving and the platform turns? Feedback on the driven wheels would help considerably though. Using that you could set up a basic PID (not sure if your basic stamp will handle it, but theres room for cheap upgrade) Being able to control a motors exact velocity and placement would help vastly while trying to balance.