This is a description of the problem Roomba's have with the brush gear case. The problem lies in hair getting in around the bearings and gears that drive the brushes.
The folks at iRobot have been busy listening to their customers and improving their product. The new generation of Roombas have a head cleaning module with a sealed gearbox. The best part is that it is retro-fittable into the older generation roombas.