I have been building self balancing vehicles for a couple of years now. These are like a Segway but designed to be ridden like a skateboard.
These had AVR based robot microcontroller boards until recently when I taught myself the basics of Arduino. This would have saved me a lot of time if I had done it earlier.
I took a board to a maker faire in the UK recently and was inundated with interest.
As a result I have designed a self balancing platform powerful enough to carry weight of a human that could be used as the basis of a Segway project a skateboard project or maybe a robot.
I have ruthlessly cut the cost to the minimum needed to make it work, and, it is now Arduino based!
Build instructions here:http://www.instructables.com/id/Easy-build-self-balancing-skateboardrobotsegway-/
People are asking me for the code so I have put it here:http://sites.google.com/site/onewheeledselfbalancing/Home/easy-build-low-cost-arduino-self-balancer
Be warned, such a device if it suddenly stops can throw you off. I cannot be responsible for any inuries. On the other hand if you keep the speed below about 10mph you can jump off at a run and be fine.