Unless you've been living in an empty desktop computer case (one of those old beige steel types) you've probably noticed the decline of the desktop computer over the last eight years or so. More and more people have ditched their bulky desktops in favor of more portable laptops. Farhad Manjoo at Slate wrote an article on just this subject and he raises some interesting, though mostly obvious points. Netbooks, tablets, and laptops are taking over the market, and surpassed desktops purchased for the first time last year. This makes sense when you consider that laptops can now have roughly equivalent performance to desktops for only a slightly greater cost and that differential should continue to shrink in coming years.
Personally, I still prefer desktops. Laptops make concessions in available ports and obviously what internal components are available and accessible, as well as CPUs and video cards. At home, my primary PC is a laptop that I keep hooked up to about six external devices (thank you USB hub) and two monitors. It doesn't move from its shelf unless I seriously need to take it somewhere. In the past year, I think this has happened maybe two or three times when I went to a friend's for some group gaming. My gaming machine is a desktop and I don't expect that to change anytime soon. I think my brother recently spent something like $4000 on a hardcore gaming laptop. I could probably build the same thing for under $2000 in a desktop and I would still be able to upgrade it later when the need arises. Of course there's that point where you have to replace the CPU, which means replacing the motherboard and RAM and a few of the other components, but I can still reuse hard drives and optical drives and mice and so on. I just can't justify a laptop for a primarily gaming-centric machine. And I think I am in the minority on this point with the exception of the enthusiast gamers who think the lights should dim when the machine is powered up.
I do have the opinion, though, that at some point, all machines will be "fast enough," whether they are desktops or laptops or tablets or cell phones, that they can run whatever is needed of them and all it will come down to is what format you want to work in. That may mean email and IM on the cell phone, spreadsheets on the tablet, and gaming on the big screen, but the machines themselves will be roughly indistinguishable from each other. I think I can live with that.