Yes and no. While there aren't any vents, the machine still vents heat through the case. It's always best to have space for air to get underneath the computer. But the heat-transfer capabilities of the plastic iBook/MacBook shells are much less than those of the PowerBook/MacBook Pro, so it's probably not that big of a deal.
When I use my PowerBook on a soft surface, the fans come on fairly quickly, so I do try to give it space to breathe. You'll probably be okay using it on a bed, but let it breathe as much as possible.