Maybe the author doesn't remember, but it was the heat issue that caused Apple to completely abandon the relationship with Motorola & IBM who were trying to jointly resolve the heat issues in the G5 processor that kept it from being usable in the next generation Titanium PowerBook. They couldn't dissipate it's heat efficiently and that prohibited it from being scaled back to operate in a notebook environment. Now that we have Intel based laptops–heat dissipation is the primary reason the whole notebook is made from cast aluminum (it wasn't really for style)–the whole case is designed to act as a heatsink–it's only going to dissipate heat if you allow airflow around it. The two thermal sensors in the case are attached to the case to monitor it's temperature. If the case gets too hot, then the fans cut on and draw cool air in through the top of the keyboard and out through the vents along the hinge.

Apple expects you to put it on a conductive surface to pull the heat away. My Rubbermaid laptop desk is made of metal mesh which allows the heat to radiate so efficiently that the desk underneath (or my legs if I'm at home in my easy chair) stays comfortably cool.

Oh and it's not the size of the application that causes the CPU to get hot, it's the complexity of the mathematical operations it's doing, so accounting applications and spreadsheets will use more CPU cycles than your email or web browser. Gaming applications where the CPU and GPU are calculating millions/billions of texelations per second generate the most heat of all…