If you've only launched OSX on it a few times, I'd imagine it was indexing your hd for Spotlight. That would of course make it quite warm indeed.
The CPU in PowerBooks has a DFS2 mode which lets it run at half the nominal clock. That in conjunction with NAP mode makes the PowerBook really cool when idle. MorphOS of course makes use of both those features. It also monitors the temperature and controls fan speed accordingly.
