If the computer was turned off and on every five minutes in a cold room for several weeks or months straight, perhaps it would be a problem. Those few moments where the electronics quickly heat up from room temperature to operating temperature put the most stress on components, but they're made to be turned on and off every once in awhile. Don't worry about it so much
Besides, since it sounds like you don't turn your iBook off, you probably do put it to sleep sometimes, right? Sleep and shutdown have the same general effect on the heating/cooling of the electronics, as the only thing kept alive in sleep mode is the RAM.