Game developers are already having problems with the expense and time required to push out the artwork for today's games. There will come a point, and it might even be here with games like Spore, where the industry will focus more on gameplay than having the most advanced games possible. Regardless, there are quite a few laptops that can play Crysis without problem. I linked to one earlier, and it costs under $1300.
/And/ with more people playing games on consoles, the need for high end PCs just to play games is less important. I think there are many factors that are coming together to make desktops less useful.
Of course there will always be those hardcore fanatics that just need the speed that only a big desktop tower can push out, but those numbers are decreasing by the month I bet.