The idea of technology replacing our need for bodies reminds me of 'The Last Question' by Isaac Asimov.
And I think that yes, post-humanism will be inevitable, and will be restricted to a part of the population, at least in the beginning.
Now, the implications of such division are, in my opinion, not only related to the concept of 'species', but more on the moral side of things. If we are improving some people, will they feel like they are better than the rest? My guess, probably yes (the human mind goes kinda crazy with power). And that could start a lot of awful things. Kind of like the movie Gattaca.
But maybe we would get along fine :D