RoombaDevTools.com is a great way to get started hacking your Roomba. You can
read a review of the RooStick (at left) over at
TodBot.com. It allows you to easily connect your computers USB port up to the
Roomba's serial hacker interface.
Other nifty products include the RooTooth, which lets you communicate with the Roomba via Bluetooth, and the Roo232 for serial communication. Their products aren't exactly cheap, but if you want to quickly get up a running with a controllable Roomba, then this looks like the best bet.