Cool. We did the same thing with our 2009 robot.
We got most of our inspiration from here =>
http://www.techunited.nl/index.php?p=11
If you mount some slides on either side of the ball you can even turn with it.
Running it on carpet changes things. Its harder to get the ball to spin on carpet, and if it stops spinning you lose control. Using small wheels instead of a board will allow it to spin more easily.