The brush sockets - OMG.
I got the screw-drivers out and sat down with the Roomba on my lap and the first thing I saw was a screw on the floor. Anyways.
So, to get behind the brush sockets you have to take the back off, then remove the tray which holds the brushes, and then remove the tray, and then remove a panel which is held in place with six screws. All those screws seem a bit much given that the panel isn't very large, but it turns out they keep a set of cogs, which connects the motor to the brushes, in place. If the cogs move out of position the brushes won't spin, ergo many screws.
The cool thing about Roombas is that they're constructed to be taken apart. There is none of that mind boggling snap together BS that you'll find in Apple products. It's all normal screws and plastic pieces molded to fit together just so.
When I'd removed the panel I found the cogs, their axises (axii?) and the motor covered, entwined and furry with an impressive amount of dirt. The brushes needed cleaning alright. I am sure Roomba's sensors kept it from catching fire from the friction.
It took some time to clean everything. I soaked the cogs in degreaser and scraped the dirt off with a scalpel (there TOTALLY are legitimate reasons for a cutter to own surgical scalpels!). As I am not the most gentle person in the world, or the most coordinated (I can place all tiny screws carefully in a bowl and then knock the bowl across the room), I was a bit worried I'd pull the cords loose or cut them, or myself, as I was poking around, but I cleaned it without incident, and even found the spot for the left-over screw.
The Roomba's been running now for at least 30 minutes so the whole endeavor must be considered a raging success.
Interestingly, a blue light I've never seen before is coming in occasionally. According to the manual it's the "dirt detect", and means the Roomba's selected that area for extra careful cleaning. I'd like to take credit for making that feature work, thanks! I bought that Roomba refurbished some four years ago. In that time I have never seen the blue light on. No, I don't think it's because it's extra dirty in here, you be quiet!