I think its OS X and where it keeps the fan speeds based on temp. I am running my MBP Penryn 2.5 on my lap. For the last hour. 26% CPU utilization almost constantly on both cores. 47 degrees Celsius and the fans are at 2600RPM. This is without any paste change out. I am not charging the battery, or running off mains. This is also running VMWare Fusion with XP pro running on another "space".
I could get it lower if I changed the fan speed to a higher RPM. Lets try that real quick. 2 minutes of the fan at 3600RPM, and I dropped 5 degrees Celsius.
I could not hear a difference, but I do have the TV on.
I use Fan Control. I don't like SMCFan Control, I do nto want to manually change the speed, I want something that increases the speed if the temp rises. It just seems more Mac like.