"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.
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... .