I've found that these Rumble Robots are easy to find on eBay and at thrift stores.
I've been hacking them since last when I was inspired to mess around with the Arduino after attending teh 2008 Bay Area Maker Faire.
Well, this year, I'll be IN the Bay Area Maker Faire in May, with my Rumble bots: Here's my how-to page on my website: http://www.dinofab.com/rumblebot.html
...and here's some videos: Part one... http://www.youtube.com/watch?v=tYRd0_7p-IA
Part two... http://www.youtube.com/watch?v=rSHpy9Yql9M