Good news everybody!
The arduino + high current motor controller shield + xbee shield from SparkFun all work together! When I originally designed the motor controller shield a year and a half ago, I had planned on using the original arduino xbee shield. Since then, I discovered the the SparkFun version which features prototyping area, option to switch rx/tx lines to pin2 and pin3, and doesn't require the programming header on the arduino for the reset line.
Although I need to do some more load testing, I couldn't help to throw on some headers and fire it up. All that is left is to add some thermistors for monitoring temperatures, throw the system in an enclosure, and dump it on some wheels and drive it around!