Actually they can catch fire, which could cause a bit of damage to the lap which they happen to be sitting on at the time.
The batteries were manufactured in the high quality factories of LG Chem in South Korea and assembled in Taiwan during that tricky time between Christmas and New Year of 2003. They seem to have come with their own internal short circuit which causes the battery to first overheat and then catch fire.
Apple said it has had four reports of batteries catching fire but no flaming groins have been reported yet.
The recalled batteries, which contain cells manufactured during the last week of December 2003 have the model number A1045 and serial numbers that begin with HQ404, HQ405, HQ406, HQ407 or HQ408.
The model and serial numbers can be found on a label on the bottom of the battery. There's a note on the Apple site here. µ