Here are the three bots I'll be running around like this, all day long.... at the Maker Faire in May.
Two have PING sensors and bump switches connected to an Arduino. The third one has only three bump switches connected to a Seeduino.
http://www.youtube.com/watch?v=1uYxWvU146I
The code for the green one is here: http://www.dinofab.com/rumble%20ping%20bump.html
::)