After reading through the discussion in Team 67's CAD thread about getting out of t-bones, I had an idea for a t-bone escaping omni drive that's simpler that has good traction, two speeds, and only requires two single speed gearboxes and was wondering if it was feasible/useful. It does have a drawback- it can't turn in high speed (but it goes sideways).
It is very similar to a normal 6WD drive, except there is an extra set of back omni wheels that have 1/2" ground clearance and the normal traction wheel is moved forward to make room. When sitting normally, it drives exactly like a 6WD. This set of omni wheels are driven exclusively from the left gearbox.
On the front of the robot, there are wheels sticking out the front going sideways. They are in a normal 1"x2"x.125" wall box, but this box can rotate so that the wheels can rotate up and out of contact with the ground. They are driven through bevel gears (or worm?) through the rotating pivot. They are driven exclusively from the right gearbox. When the wheels are rotated down into contact with the ground so that they are are perpendicular to the ground, the front of the robot is raised up off the ground an inch or so. Now, the robot is on the front sideways facing wheels (omni wheels) and the back pair of omni wheels. When the left gearbox is driven, the robot moves forward/back, and when the right gearbox is driven, the robot will move left/right. You could possibly set these gear ratios to be faster than the 6WD so the omni wheel configuration will act like a high gear.
Like this sketch, but with the big omni on the inside of the frame.
http://i.imgur.com/HGrSeNq.png