The Segway is basically a standard inverted pendulum problem with a couple of new features put in. The reason I say it is a "standard" problem is because this is a problem that EVERY student in control engineering studies in their first control course. This is the most common problem of making an inherently unstable system stable through the use of automatic feedback control.
There is a great inverted pendulum tutorial on the web that was created by the University of Michigan, Carnegie Mellon University, and Ohio State Universtiy. You can see the tutorial at:
http://rclsgi.eng.ohio-state.edu/mat...nd/invpen.html
If you plan on making any self-balancing platforms, I would suggest that you start by looking through this tutorial.
Have fun!
-Chris