People have been hacking iRobot's Roombas almost since they came out, taking them apart, reprogramming them, adding more sophisticated components. I remember reading about one sacrificial effort where some guys took a Roomba, reworked it a bit and used it to play a real-life game of Frogger. I think they made it across the street and back once or twice before the Roomba got squashed.
A little while ago, the iRobot guys opened up the interface for the Roomba. Now iRobot has a new platform just for those robot hackers out there, the iRobot Create. You've got access to all the sensors, servos and motors, port interfaces for attaching other hardware (with the optional Command Module) and all the software you need to program the sucker.
Sounds like fun :)