I've been working on a Rover 5 platform that I go from sparkfun.com. There are a few different variations, but the '5' has 4 motors, each with it's own quadrature encoder, and is tracked. Since it does have independent 4-wheel drive, you could use mecanum wheels for an extra level of maneuverability.
Thanks for the suggestion! I like that platform, but I'm a bit concerned that it would be too small (like the DFRobotShop Rover). The Chumby takes up a lot of real estate.
But I could perhaps mount two decks on top of the robot, a bottom deck for the Arduino, motor controller etc. and a top deck for the Chumby and a camera.
I've got the Rover 5 and I'm pretty happy with it, here's my setup:
If you put a larger panel on the base you'd have a fair bit of space to play with.