How about
this thing ?
I think it's a good idea that could use some refinement. You get the best of both worlds, push and omni. I think mechanum was the new thing on the scene last year, and it's still in its baby stages (Hey Andymark.biz - Mechanum Wheels... hint hint!!!). Mechanum makes a lot of sense if you want the holonomic effect with better traction and less complexity in the chassis design, but still boils down to the "makeability" of such a thing.
People have done a wonderful job extracting maximum efficiency from all the given kit components while applying them effectively. The innovation levels have been tremendous. Necessity will bring innovation. I think we've tackled most of the difficult aspects of a carpet competition robot as far as the drive base goes. A lot of innovation gets overlooked in the manipulators. But makes no sense to design to the unknowns.