You know ... it wouldn't surprise me if we do get to a point where all "PCs" are basically smart phones, and your traditional desktop computer is just a docking station for your handheld which connects to a monitor/keyboard/mouse/speakers/extra usb slots. That might actually be kind of interesting.
But, either way, big iron (read: mainframes) will still be there in the background, holding everyone's data. And likely still running programs that were written in the 70's.
I'm actually looking forward to computing in general taking a more modular approach. I like customizability and minimalism.
I'm personally already heading in this direction. I have an Acer W501 tablet running Windows 7. First thing I did was replace the Windows shell with a very stripped down Emerge Desktop and a Rocketdock. Then I moved all of my media (music, videos, photos, books, etc) from my desktop computer's hard drives onto a portable one, so that I don't have to bother synchronizing or streaming anything.
With the keyboard docking station, it's basically a netbook. The SD card that lives permanently in the device is large enough for some small indy games or emulators, productivity software, and hand-picked media, with room to spare. I got a cleverly designed small, flat-form mouse that makes it usable for any PC task that its hardware can handle. I can draw with an Adonis Jot stylus. The portable hard drive makes it a hardcore media station. My headphones are the in-ear type and fit in a tiny pouch or I have a portable mini-speaker that makes it actually enjoyable for playing stuff out loud or I can plug it into my tv, which I've used to play movies or introduce indy games to a crowd of friends.
I basically have a full desktop set-up that carries easily in a small messenger bag with tons of room to spare, and whenever I use it, I only have to bother with the stuff that I'm actually going to use at that moment. The final step is for it to be able to dock into my tower at home for a boost to desktop performance on one OS install.
The whole raspberry pi thing is one more indication to me that this is going to become a more supported way of doing things over time.
Software is doing it, too. 3D art used to be done almost entirely on huge, bloated, inefficient all-in-one packages like Max and Maya. Now there are specialized softwares out there for every individual function that those big packages perform. Theoretically, I could zbrush sculpt on my tablet, and transfer the models to my desktop remotely to perform hours-long renders for me while I'm away.
What bothers me is the majority of these devices aren't currently being designed for the kind of flexibility I'm taking about. They're being designed like shiny expensive toys. When I was shopping for my tablet, I was shocked at the sorts of criteria I read through hundreds of reviews on various tech sites. Apple is pulling the industry around by its ass, and it's infuriating. Everything is judged by how stylish it looks and how small it is, not by functionality. I did massive amounts of searching, and out of the many dozens of devices out there, only a handful even had fully functional usb ports... it fucking blew my mind. A single, tiny USB port offers untold worlds of possible function, but the devices that included them were constantly put down for weighing some 1/20 lb heavier than the competition, which is just too heavy to be worth it
... and don't even get me started on the damn capacitive screen fad or the general hatred for stylus. I honestly don't understand how people do anything with their iPads that could possibly make them worth the investment. I've looked hard at the ios and android devices, and the sorts of compromise and work-around you have to deal with in order to do anything other than consume media and play cutesy flash games would give me an aneurysm. I have trouble believing that the market for people who actually want capable hardware is really that much smaller than the lolcasual user market, but whatever. This is just pet peeve.