Sure, but "post" implies replacement, implying that PCs are somehow obsolete. It would be akin to heralding the introduction of compact cars to being the "post-sedan" era. "Mobile computing" is a more accurate and less marketing-driven way to say "post-PC". Mobile computing hardware, like the Palm Treo, has been around for awhile... 3G is the technology that has really helped it come of age.
Post PC means Jill Sixpack always has a net-connected device within arms reach. This is a big step closer to constant personal net connectivity.