It's a "problem" that i have since 11.10, now I've upgraded to 12.04 (fresh install). Even while ondemand governor is set, and the computer is idle, fan goes on and doesn't want to shut down.
Open up the terminal, do a
and type in:
dstat is a system monitoring tool (like vmstat) and will show you the processes which consume the most CPU resources.
Had the same problem. I had to remove the following boot-parameters (which I have added some months ago) to solve the problem: