When I first started analyzing the game I figured that a basic 6 WD robot with 8" wheels could easily climb the bumps. In this configuration there is no bottoming out issues, and the CG can be kept quite low.
However, we discovered a critical flaw when we tested this.
When a 6 WD robot climbs a ramp*, the center wheel acts as a pivot as the robot climbs over the crest. This results in
half the robot being lifted into the air 
Highly unstable!
Take a look at the demo bots in the game vid
(The non-animated ones of course!). They all have
FOUR wheels. This is a much more stable configuration b/c all four wheels remain in contact as the robot climbs. There are no sudden rocking movements forwards or back.
Of course, as Dean himself pointed out, 4 WD robots don't turn very well...
*or a tracked drive