My iBook got up to 60 degrees C today, which translates to just under 150F. My fan came on when it hit around 58C, and stayed on for about 15 mins. The reason mine got so hot was because it was almost buried in bed covers, so it was getting no ventillation. I think these temps are fine though - if the fan just comes on at 58C, it must be fine for the book to run that hot, or esle the fan would come on much sooner.