There is no fan in the iBook. There are simply slits on the top-side of the case that allow the hot air to escape. Closing the book would cover these vents.
No fans for the win! And it still only manages to get a bit warm on the underside when playing YouTube in mobile mode. If you put an SSD in the machines, they become completely silent.
Also worth noting, I'm not sure if the video card is partly to blame. Most of the Clamshells didn't even have a video out connector, and when they finally added the feature to the SE models, it was through a composite connector/adapter that you plugged into the headphone jack.

Since we're posting screenshots