One word..
..Games.
I believe that's what has really been driving the increase in power in PC desktop hardware - just try playing FEAR on a terminal or a 386.
For the forseeable future gamers (and those who suffer from crap BT DSL-plus latency) will still want/need desktop machines with a load of processing power and quality, secure, efficient operating systems and the applications to go with them (or Microsoft Vista and Office).