Yesterday, I came home and the fan was on high speed (I could tell by the loud noise). I found no stuck processes. I rebooted, and the fan noise was still high speed.
I opened up the case and vacuumed it out (not a lot of dust; certainly nothing caked). I scrubbed the fan blades and blew out the hit sink below it. Upon reboot, the fan was immediately loud. Cold machine, fast fan; something's busted....
One perhaps unrelated incident is I used Registry Mechanic to clean my Registry last week. Doesn't seem to be any direct tie in to the fan, but I throw that out, just in case.
So is this a fan replacement? Or is there some thermal control that has gone wacko?