This is last years information, but I ran accros a reference, and do not remember seeing anything about it here. 40 minute video. Would be fun to have one of those PR2 robots to play with!
The talk mentioned being able to use ROS with Arduino, so I looking up this information:
Not enough to be able to build a complete bot running ROS on an Arduino, but enough to make it easy to use Arduino as part of a bigger system using ROS. A much more complete implementation will run on Andriod [in Java].