Evidently, the iRobot home robots (the Roomba and Scooba varieties, with emphasis on the former) are now being used as robotic development environments.
It makes sense. They're tiny. They're robust. They're packed with sensors and motors. On top of all of this, they've gone through a modern manufacturing process so they make for a nice little package. On top of all of this, iRobot provides a serial interface by way of a 7-pin mini-DIN on top of the unit (this pin-out is the same as an 8-pin mini-DIN, so it's not so difficult to find a cable to work with it).
How do you use all of this? Well, iRobot has a developer page to get you started. From there, you can find information about the serial command interface (SCI). From the resources there, it's pretty easy to hack together serial interfaces to get up up and running programming the internals of your robot . . .
. . . HOWEVER, RoombaDevTools has done a great deal of the work for you. The two apps that I think are the coolest are the RooStick and the RooTooth.
So RoombaDevTools gets you right up and running with documentation, hardware (including cables), and software as well as lots of other helpful stuff. PLUS most of the importnat software is either platform-independent (i.e., Java libraries made for the JVM) or provided for OS X, Windows, and more. IN FACT, a common application is to combine a Roomba with a control system built on top of a GumStix. That's WAY cool.
So there ya' go... The Roomba(+Gumstix, perhaps) -- the next affordable robot development platform.