I'd rather use 4 omni-wheels. It would take more weight and all that but a rectangular chassis is easier to fabricate and deal with than a triangular chassis.
http://www.chiefdelphi.com/forums/sh...?postid=420541
To drive forward, all wheels drive forward. To go to the side, the wheels on the same side will drive in opposite direction. wheels in the opposite angles of eachother will drive in the same direction. To go left, the front right and back left would drive forwards and the front left and back right would drive backwards. This would be opposite to go right.
I'd expect that if you were going in a direction parallel to a pair of wheels ie. Front left and back right, then the other two wheels would turn at all.
I hope I didn't make that too confusing