We had a 6-wheeled robot this year...I don't know whether it was six-wheel drive though. It took us a few days to get the middle-axle lowering set the right way, but it helped a LOT. Oh I should tell you that we used some spiffy donated tracks, and it made us look like an R/C tank out there.
Robo pics and videos are here:
http://www.sau53.org/net9/First/media.htm
You may also see the team at PARC in May. I wish I could go, but I'm going to Washington D.C. that weekend :-C