"The post PC era" is meant to refer to a world where (except in some specific professional contexts) people largely stop buying PCs because of a new class of mobile devices that takes over their workload entirely.
Then I think we're a long way from seeing this era, simply because anything that requires more than a couple lines of text isn't just better on a keyboard than other input devices—it's vastly, insanely better (at the moment), a problem I don't see being solved in the nearish future. Rather, I suspect many if not most people will have one PC-esque device for keyboard heavy things and one or more "mobile" devices, using your definition.
This also assumes you're not counting laptops under your "mobile" rubric, which I don't think you are.
"Then I think we're a long way from seeing this era"
Agreed, but we're just starting, maybe in a few years, who knows...
IMO, the term post-PC is a nice way to differentiate Android/iOS/WebOS/etc tablets from previous generation tablets that ran Windows or some Desktop version of Linux.
I think the laptop, rather than tablet, form factor is going to dominate in the medium-term future, because the portability vs usability tradeoff just doesn't encourage giving up your keyboard.
That doesn't necessarily mean that the laptops of the future will have a full-featured OS rather than a locked-down iOS-style OS. I mean, I'm sure yours will, and mine will, but will your mother's?
They'll just put their iPads into a keyboard dock, or bring a keyboard attachment like the HP TC1100 had. The keyboard could be something like 100g by itself too.
Then I think we're a long way from seeing this era, simply because anything that requires more than a couple lines of text isn't just better on a keyboard than other input devices—it's vastly, insanely better (at the moment), a problem I don't see being solved in the nearish future. Rather, I suspect many if not most people will have one PC-esque device for keyboard heavy things and one or more "mobile" devices, using your definition.
This also assumes you're not counting laptops under your "mobile" rubric, which I don't think you are.
Anyway. I don't even see desktops going away all that soon because of one thing that gets lost on HN discussion: they're cheap: http://jseliger.com/2008/12/26/computer-post-desktop-or-lapt... .