I've heard several people mention this problem, including Sheldon Brown. When narrow rims are used with fat tires, and the rides are long and gritty (as on long tours over bad roads) the problem tends to be worse.
I'm wondering about possible solutions. Somewhere I read about sidewall protectors that can be installed between the rims and the tires, but I'm not sure where they are sold.
Also, there may be other solutions (aside from wider rims and narrower tires). Does anyone know how this problem can be prevented or slowed down?