6WD. Drop the center wheel 3-4 mm, nothing but traction. The crush of the carpet makes the rock almost imperceptable. This was the drivetrain that 1189 used this year. I think the folks at Boilermaker will attest both it's pushing power and agility (for a 2 degree of freedom drive system).
http://www.gpgearheads.org/gallery/album08/IMGP1542
Don't have a good picture of the finished drivetrain, but the finished version had 6 8" AM Performance wheels with roughtop treading.
I recommend that all teams (especially those having difficulty turning) reference the following white paper:
Drive Train Basics (How to Be Sure Your Robot Will Turn)
http://www.chiefdelphi.com/media/papers/1443
And the spreadsheet derived from it.
http://www.chiefdelphi.com/media/papers/1917