Hot enough to be uncomfortable? If it was being used heavily (long periods and/or lots of processor activity) then this is probably normal.
Hot enough to burn skin? Be concerned.
Hot enough to burn or scorch nearby objects? Unplug it now!
The PowerBooks seem to dissipate heat through the bottom plate. If it was sitting on a blanket or something, it might not be able to keep itself cool. Try putting it on something that allows air to circulate underneath.