excuse the long post, im bored and wanna type.
As far as games: PCs dont really have a wide variety of games out there (MMOs, FPS, action, Strategy, cards, etc). When a new game comes out for the computer, it isnt going to get as much publicity as lets say, Grand Theft Auto: San Andres. Then again, when a game from a platform is ported to a PC because the company thinks "Oh, we sold lots for ps2, why not try computers?" thing is, all the people that would play it on the computer, already have it on PS2/GC/Xbox. How many of you actually played through the computer FF7?
Also, it depends on your gaming habbits. A person that his been playing on a nintendo system for all his life isnt going to all of a sudden start playing a PC. Sure, you can buy a controller for the computer, but its still an akward fealing sitting behind a desk, monitor, whatever.
Another point is programability. With platforms, you are given a certain system specification you can program to, where as w/ computers you have to consider your average joe E-machin all the way to your super duper 2000+ gaming rigs.
And no offence to stupid ppl, but anyone can play a console, you just put in the disk and go, whereas on a computer you have to install, update, tweak, etc. So by having a simple system you dont have to scare ppl away with "technology"
As for the systems: I think that platforms are going to gain some popularity. With Xbox Live being such a hit, i think a lot of the companies are going to start trying to go online, cuz it offers more depth to your games, and an extra source of monthly income from the consumers. Also, graphics will be better, so your brand name games: legend of zelda, metal gear solid, halo, etc are almost guaranteed to come out and sell well.
But the PC shouldnt lose any momentum. Like many of yall said, PCs change very quickly. Sure your graphics arent as good, just wait a few months till the next batch of cards are out and kick the platforms' asses. Most ppl will buy a platform when they come out, but they arent going to ditch their PCs! But ya, there needs to be something that introduces computers into the mainstream gaming scene, because there is so much potential in a computer's abilities, just the ordinary public just needs to see it.
_________________ 
|