A commenter in a MacRumors thread about the subject took screenshots from Apple's site, showing they had removed the phrase "Push happens automatically, instantly, and continuously." In its place, Apple clarifies that changes from the iPhone and web apps are updated instantly on connected computers, but not vice versa.
Apple also notes in a Knowledge Base article that changes "made on your computer will be synced to the MobileMe 'cloud' once every 15 minutes (or every hour in Mac OS X 10.4.11)."
That, to me, doesn't sound like "push," it sounds like "sync." The term "push" still applies, however, to me.com email, as that shows up immediately (in my experience, at least.) Calendars and contacts, though, not so much, apparently.