Obviously, I don't know what goes on in Cupertino, but it's clear that Apple had a plan and they seemed pretty confident (re: the announced September date) it would work.
I think they jumped too soon on the idea. When Jobs first described it, it was clear to any senior developer (who's had to do this kind of thing before) that it was not at all well thought out.
The entire concept has communication state holes in it. Developers would need their own servers for even stupid little apps. The background push would chew up battery for many users. And yes, not to mention the need for 100% reliable Apple servers, something they're probably not ready to provide.
They might go ahead with it just for IM apps, though, and to save some face.