I've noticed that the fan's in the aluminum macbook don't spin up really till about 90C... then hold the temp around 80C (under full stress testing in windows)
Not having one I can't comment. I can, however, say that the operation of the fans under Windows and Mac OS X may well be different -- I'm pretty sure they're different on the pre-unibody MacBooks.
I'd find it kinda strange if the default acceleration curve kept the fans off until 90C -- I can't imagine that's good for the lifespan of the components...