I only notice the heat when I use my powerbook in bed watching a movie or something. Sometimes when it is in my lap. The fan spins up fairly loudly, but since it is no longer my primary computer I don't do much heavy usage on it anymore.
But yes, I have heard on these forums that Powerbooks run hot! There are some programs that will monitor temperature, and I think if you search the forums you can find out typical temps for other users?