In a decentralized system like email or jabber there is responsibility, @example.com is responsible for delivering messages to its users, and for the necessary communication with other servers.

And of course you'll want to use push messages for communication between servers. It really doesn't have to add all that much latency, a few roundtrips is perfectly doable. That's a very small price to pay for the benefit of being independent of the whims (or unreliability) of a single entity.

That said, it's probably not going to happen soon, but not because of technical issues. I think it's just a matter of inertia among users, just like people are still using MSN messenger or skype instead of jabber.