After receiving "Spot", my Roomba last Christmas, I have been really hankering to hack it. After digging around forums/mailing lists/google for awhile, nothing was yet available ready to use.
Several months ago, iRobot, the manufacturers of Roomba, released OSMO Hacking Module for interfacing with the Roomba's onboard serial port. Since then, there have been some very cool projects that have cropped up; everything from a long serial tether, a bluetooth interface, a programmable plugin module, or a full on-board computer with WiFi access.
I think I'll go for that latter. AutomationWiki: Roomba Entry