In his blog, Michael describes a nice 4WD robot he realized by means of an arduino-compatible board, a motor shield and a couple of XBee radios, which have been used to implement a simple and effective remote control.
Actually, the remote is made up of a standard breadboard equipped with a joystick, a couple of buttons (that can turn the robot in a Kitt-like vehicle!) and the XBee radio. One interesting feature of this project is that the remote controller is fairly simple and has been designed to work with just the XBee radio board, instead of requiring an additional MCU.
More details can be found here.