We now carry the Arduino Robot, the first official Arduino on wheels, which includes a pair of ATmega32U4-based platforms that control two motors, sensors, and more. From the Arduino blog post about it:
Designed in cooperation with Complubot, 4-time world champions in the Robocup Junior robotics soccer, the Arduino Robot promises endless hours of experimentation and play. It is a self-contained platform that allows you to build interactive machines to explore the world around you.
You can use straight out of the box, modify its software, or add your own hardware on top of it. You can learn as you go: the Arduino Robot is perfect for both the novice as well as those looking for their next challenge.
For more information about the Arduino Robot, see the product page.