Just one more thing : considering PC's CPUs, in order to work (switch) faster, they must get smaller*. The number of transistors and their switching frequency increases, the surface decreases, therefore the heat density rises. So it should not be suprising, that modern CPU can have larger dissipated power density than the iron !
* - But in general, all dependencies are much more complex.
As long as CPU's performance depends on their switching frequency, one must reduce their size to avoid occuring of so called 'transmision-line phenomena'. However, it's not always like that - if you want to know more, You need to read something about semiconductors and computer architecture. Just to give some inspiration - there use to be an architecture called GAPP, which could solve certain problems faster than any other, while switching only with frequency around tens of MHz. And most circuits now works according to so called synchronic paradigm, and there's lot of interest in asynchronic ICs, which - while difficult to design - would bring heat related benefits.
I hope this gave some food for thought - I wrote it to be somehow useful, as so long I only ask questions here :?