Again, I'm not saying this is a design that's right for FRC, I'm just throwing it out there because I'm doing it for a mini demo robot. The single 775 design would never be used for FRC. This robot fits in a cubic foot box. My friend came up with the idea, and I was a little skeptical that it would work for the mini robot, so I searched and found this video, which I thought was cool.
As shown in the video, it is able to go sideways, but I believe that the gearboxes used in the real videos do not allow rotation like that, so it is only shown rolling sideways down the hill.
To answer Cory's questions, I think the advantage would be a drive that allows the robot to slide to the sides with (possibly) more traction than mecanum, but I'm not sure how much traction this would actually have. It's also simpler in the fact that it uses fewer motors/fewer gearboxes.
Teams probably haven't done this because there is a good chance it will not work very well, so they didn't take the risk. It is not extremely easy to make the parts, and I doubt any team could get the design right on the first try.
As for the surface, it does appear to work on solid ground in this video, so it may not need a deformable surface.
http://www.youtube.com/watch?v=3uftJ3JJQk4