Have you guys thought of a six wheel drive wide base? I modified a kitbot at the start of the season to do this (1/8th in drop on the center wheel). I haven't gotten to drive it extensively, but it seems to work.
As for four wheel drive. We have successfully gone two traction wheels in the back (or wherever most of your weight is), and omnis in the front. I can't find a vid anywhere, but those at bunnybot 2010 can attest to the fact that it was quite strong and fast (we were able to tow a goal that was made out of rough plywood on uneven carpet and win a few pushing matches). It was also very easy to drive (we managed to deliver coffee without spilling it
).
If you don't want to modify your bot to try it out, you can build spacers that allow you to put two omnis on one hub, allowing you to simulate a full width wheel.