Thanks ScrubJ, that's too bad about ceramic not solving the problem. Although maybe I should be glad - no need to go spend a couple hundred bucks on a new BB.

I did clean it about as well as I know how. Degreased it with alcohol, letting it soak for quite a while, then dried it out before reapplying grease and reinstalling. The BB only has about 3,000 km on it, but I guess it's possible there was something wrong with it from the beginning. But would the problem then go away for a short while after servicing it, before the creaking comes back?