So there was dirt everywhere, and I submitted a ticket on the iRobot site to try and figure out whether the robot could be fixed. They promptly (within 2 hours) sent me a troubleshooting list, and also added that if I could to fix the problem using the troubleshooting routine, I should send them my name and shipping address.
The robot successfully failed my troubleshooting this morning, so I responded to the ticket and gave them my address.
What do you know? About two hours later, I got a message saying they have shipped a new cleaning head module to me.
Just like that.
No payment. Under warranty.
Mind you, I purchased the robot in the Fall of 2007.
Hat off, iRobot support.