It's interesting that with all the rise of tablets, smartphones, netbooks etc you keep getting articles in the mainstream press about the 'death of the desktop computer'. I think what is starting to happen is that for people who aren't actually at all interested in computers as such, they will more and more use other devices for their online shopping, banking, social networking, etc. Meanwhile people who are actually interested in computers 'proper' will carry on using them. In some ways it will revert back.
I think we *will* see the death of the "desktop" computer, just not as quickly as they think.
What's important to realize is that this is the death of the *form factor*: Eventually no more big bulky boxes that's stuck in a single location.
My cellphone is now far more powerful than my desktop from just a few years ago. New cellphones come with HDMI out in many cases. Wireless HDMI is on the horizon. Bluetooth keyboards and mice. In a couple of years time, for *most* people, a monitor and keyboard with wireless connection to their phone will be sufficiently powerful to do whatever they want. Even for hobbyists. Even laptops are being threatened by this and pads (see for example the Asus Eee Pad Transformer, and whatever that other one that was announced that was a screen, keyboard and extra battery that you slotted your cellphone into to use as a laptop)0
Even in the "do it yourself" arena, look at the Natami and Replay boards. See how tiny they are. I don't remember the precise dimensions of the Natami, but the Replay is about 1/3 the size of a MiniITX board. Another few years and you'll probably be able to put together powerful FPGA powered machines small enough to fit the shell of a current cellphone.
Storage is rapidly exceeding what most people need in tiny packages. My phone has 32GB, and that 32GB card is smaller than the nail on my little finger. Higher capacities in the same form factor are on the way. If I want to store movies I can buy several TB of storage in an external drive smaller than the smallest desktop I've ever owned, and it's getting smaller (or bigger capacity, or both).
Give it a few years, and 99%+ of computer users will have their computing needs *and* their storage needs met by computers smaller than a cellphone, and will only have larger form factors when/where the form factor makes a difference (e.g. tablets for the large screen; laptops or "laptop shells" for keyboard/screen combo) or if they have specialist needs or are being contrarian.
At that point, does it make sense to talk about "desktops"? If the only connection your computer needs is the occasional charging cable, and it fits in your pocket, why chain it to your desk? Maybe you'll keep a large screen and a keyboard at your desk, but no reason to leave the computer there.
The remaining 1% or so (of course the actual percentage is just a wild guess) will still have plenty of opportunity to buy boxes to stuff full of components, but your needs are going to have to be pretty weird for you to need them, and even then you might find that such needs can increasingly be met as "expansion boxes" wirelessly connected to your machine the way network storage can be today.
Doesn't mean that's what they'll all buy or have that quickly - it'll take at least a decade, probably longer, to shift desktops fully out of the mainstream, but it's pretty much inevitable that it'll happen.
But keep in mind that desktops are *already* on the way out. They make up a rapidly shrinking minority of computer-like devices consumers buy, between laptops, netbooks, smart phones, pads etc. And the ones that do get sold are shrinking and/or getting baked into the screen. Last time I went to the local PC World store, I don't remember seeing anything larger than a Mini-ITX style box, and the most space was taken up by laptops and combined computers and screens and cellphones. Of course you can buy bigger ones still, and will "forever", but that's increasingly being relegated to specialist stores.
That's not to say that lots of people won't have computers they keep in one place, but mostly for convenience rather than any need to have boxes big enough to not fit in their pockets or bags.