100% CPU on idling

[Gianluca Sforna]

I am curious if anyone else is seeing it's processor at 100% when the
machine is supposed to idling (I do not use fancy screensavers, just
go to a blank screen after 10 minutes).
I noticed because this is a laptop and the fan is triggered by cpu
temperature raising.
As soon as I touch the mouse, the CPU stops processing, but I can see
it was at 100% in the system monitor applet.

I suspect this is the inteded behavior of something but, nonetheless,
I would like to know what actually is (and eventually kill it...)

cheers

Gianluca