Honestly, I'm of the mind that the last ten years has been one big bubble for desktops, simply because we were making them do things that they really shouldn't have been doing. Until recently, our laptops sucked, virtualization technologies were poor, and graphical horsepower was always struggling to cope with our available resolutions and gaming engines. Now, we've got laptops and netbooks (and I believe even Intel granted that many netbook sales weren't "new" sales, but "instead-of desktop" sales). Virtualization setups can drastically reduce the number of real desktops in business', while our multi-core chips have more than enough horsepower to handle an office full of machines that do nothing more challenging than WYSIWYG. And even mid-range GPU's can usually handle games on medium detail.
Honestly, we don't NEED to buy new desktops anymore. The economy has likely made us realize that a little sooner than later, but I think people are really cluing into the fact that they don't need to upgrade, and that many upgrades have very dubious benefits.
I think companies are realizing this too, to a certain extent. Look at Nvidia's push for non-gaming applications. I believe OCZ was recently quoted that their SSD's sales are (or will be) their number one product now. In fact, it's amazing how many memory companies have suddenly jumped on the SSD bandwagon - it's irritating in a way, but I think they're all seeing the writing on the wall. When the latest generation of DDR3 sticks are exceeding JEDEC specs by 100%, you know you're selling luxuries, not needs.
Case in Point: I just recently reloaded Crysis:Warhead, and played through the entire game at 2560x1600, Gamer quality, with absolutely zero problems, with just a single GTX280. And over the next half year, Nvidia and ATI are going to try and sell me a bigger card? Oh, they'll succeed eventually, but they sure won't catch me lining up to pay the Day 1 price-gouge.
And for aftermarket stuff - even watercooling is hitting a bit of a stall, because the benefits just aren't what they used to be. My i7 920 honestly doesn't care if it's at 60*C or 80*C, my GPU is the same at 35*C or 55*C. It's fun, but I don't delude myself that I'm getting any extra performance because of an extra few degrees. That, and the used market is really coming into its own these days - a little patience and research, and all you need to buy new is tubing and clamps.
My 2c.