It's a configuration that worked great for us in the past.
http://www.chiefdelphi.com/media/photos/13930
The center wheels were pneumatic (air filled) while we had 4 casters on the 4 corners of the robot.
Be careful as to where you mount your casters in relation to your center wheels.
You more than likely want all 6 wheels touching the carpet on the same plane, so mounting the casters lower (if they are smaller than your drive wheels) is necessary.
The bad thing with this design is that your robot can and will be able to spin very easily.
Your turning point on your robot will now be directly in the center of the chassis (re: right in the middle of your center drive wheels).
Cool for "crazy death spins" but not so cool if you lose your bearing as to which is the front, and have to turn a lot.
I personally think that design would be sucessful as far as a straight lap robot.. (ie: one meant just to drive around and do laps) but not for any robot that wants to carry or hurdle a ball this year.