Here's what were doing.....
http://www.youtube.com/watch?v=K3Nb9NAxdeE.
We are building a full scale prototype with Banebot 20:1 26mm gearmotor and their 3 7/8 30A green wheels. Our students have played with this concept using one 20:1 Banebot gearmotor and a soccer ball. That playing was promissing enough for us to order the parts for a full size prototype that we are going to mount on our prototype holonomic drive.