One fundamental limitation you didn’t mention is user interface. Oh, things keep getting better (see iPhone’s multi-touch), but these devices are still going to be limited by having screens small enough to fit in a pocket and either a stylus (which can be a little cumbersome and is easy to lose) or a touch interface (which takes up lots of screen real estate if it’s to work with the thick-fingered public). There’s a reason we still use the keyboard and mouse - they’re effective in a wide variety of situations.

I didn’t bring up that point because McCracken addressed it. It’s his position that we will be able to conveniently hook up a monitor/keyboard/mouse/etc to the device.

We’re not that far off from that, actually, but what you’re still left is a slow processor on a device with limited memory, hard drive space, and multimedia support. It’s possible that some people would prefer that for the ease of being able to carry their computer around with them. I’ve had ideas along those lines myself. But it seems to me that it would be in combination with, not replacement of, a standard PC. And if you’re going to hold on to the standard PC, it might make more sense and be simpler to let me simply plug by device into a PC and have it share resources, acting like a hard drive and emulated platform so that you can run the smartphone apps on the computer, access files, and so on.

Another issue is operating systems and compatibility. At least as of now, mobile devices need to use ARM-type processors if they’re going to 1) have a small enough battery to remain pocketable and 2) have enough juice for an 8-hour day. Hence PDAs and smartphones, which need applications specifically developed for them. Then there’s ultra-mobile PCs, which can have access to vast collection of programs written for Windows, but they lack the UI. Intel’s Atom and it’s upcoming chipsets to pair with it may bridge this gap somewhat, but you’re still stuck with a UI/platform problem. In order to be effective as a smartphone, someone’s got to define some basic parameters like screen size so that everyone developing for the touch/stylus UI can make it work. To perform all the functions of a modern-day laptop, hooking up a keyboard, mouse, & bigger screen is well and good, but that would require a substantially different UI. The iPhone has one, PCs (Mac or Windows) have the other, but no one has merged them yet. And the combination of this with the hardware problems you mention make me think that widespread consumer adoption is still a decade off.