Any device which is A) personal and B) computes is a PC, to my mind. The argument that phones and tablets aren’t PC’s because they’re less capable than desktops is more than a little specious. Our phones and tablets are arguably more powerful and capable than our desktops and laptops were ten years ago–with a few exceptions given for things like video editing, but even that task was pretty primitive back then compared to now. If tablets aren’t PC’s because they’re less capable than a full-blown modern Mac Pro, should we also retroactively decide that 1995’s computers aren’t PC’s?
The idea of “what a computer is” isn’t changing, per se, it’s just that we have this arbitrary distinction between mobile and not-so-mobile. That distinction will continue to erode. I have no doubt about it. We’re talking about form factor here less than we’re talking about essential functionality.
Back to the oft-repeated notion that one “can’t get anything done” on these mobile devices: I personally own a Windows desktop, a Windows netbook, a CR-48 (chromebook), an iPad and an iPhone. Any computing task I undertake–anything, except for video editing–can be done on any of these devices (nota bene: I’m a professional filmmaker, so the editing I do is of a different sort than that which Joe Blow is going to do…and that can be done on an iPad, pretty much). The idea that “real work” can only be done on a desktop or a laptop is incredibly fallacious, and I think the people who say that sort of thing are probably most often people who don’t (yet) own tablets.