Thanks for the replies, that clears that mystery up.
bandit - Do you suggest replacing the sprocket just because the nylon has broken up? I see no reason not to do it at the same time, but if I change the front sprocket I'll be tempted to do the chain at the same time, then the rear sprocket too (both worn, but not enough that I'd normally replace them yet)...
As for nylon vs. plain steel sprocket, does the nylon ring make much of a difference? I'm guessing it only has an impact when switching between acceleration and overrun since the teeth would keep the chain loaded sideways the rest of the time?