Is a TV a Computer?
About a year and a half ago, I was talking to a guy from Samsung who indicated that Samsung intended to start putting ARM chips in TVs, and was considering running Linux on TVs. If your TV has a desktop operating system and a fast CPU, is it a computer?
Set-top boxes like the Apple TV are blurring this line even more. The Apple TV runs OS X, but with a limited suite of applications. Apparently you can put all of the missing bits back in, ending up with a moderately fast computer connected to your television. Add a keyboard and pointing device, and what's the difference?
These things are quite uncommon at the moment, but when every house contains a TV that's really a computer, there's a lot less reason to have your own PC; you can simply log into the nearest screen and start using it. This is the first step toward what's typically called ubiquitous computing—the step after desktop computing, in which computing resources are so common that you don't differentiate between consumer electronics and computers. Some problems related to data access—and, most importantly, security—are still unsolved, but when these issues have been addressed, when do you need a computer?
The obvious answer is this: When you're outside. No matter how ubiquitous computing becomes, I can't imagine the idea of installing terminals in the local botanical gardens being too popular, and yet that was where I chose to work for a lot of this summer (although, this being Wales, summer lasted only a few weeks this year).
If a portable is something I carry when I'm not near a fixed computer, then it has to be able to fit in my pocket. If it isn't, then I won't carry it at the times when I most need it. The question then become how I want my screen and computer to interact. Being fixed, my display may have a faster processor than the device in my pocket (at the very least, it doesn't need to worry about battery life issues). I could use remote X11 to export my display from the pocket device, or I could treat it as a dumb display and connect something like an HDMI cable.