Great fun. :)
You want to build a bristlebot yourself? It's pretty easy, here's how.
To build this bristlebot, I used one of these nice toothbrushes. (I'm not trying to promote Oral-B here. You can probably get similar toothbrushes from other vendors as well.) They come fully equipped with motor and batteries, for about 4 Euro at the pharmacy. Apart from that, only some adhesive tape and strong scissors are needed.
Update: There's another nice bristlebot on Sebastian's site.