I took my black MacBook to the Genius at the Apple store (Oakbrook, by the way) to get this fixed, and his response? "It's normal. A fan will do what it needs to do to keep the machine cool." Gee, thanks.
As you may remember, I had a similar issue with my MacBook Pro. A new fan unit later, this problem was fixed, which certainly leads me to believe that this isn't normal behavior. (I also showed the problem to a friend of mine who is an engineer, and he confirmed that this isn't normal.) I've called AppleCare (an almost guaranteed better experience, every time) and a repair box is on its way. Sigh.
Anyone else have this problem?