Schaller: The long journey towards good free video conferencing

Interestingly, the GTalk client appears to use some entirely custom protocol. It's some sort of best-effort sync-as-you-can thing, like the GMail client, not XMPP at all. (You can see if you send messages via another client, they magically appear on your phone in real-time as if you were typing them there, which I'm pretty sure XMPP can't do.)

I assume that's their magic - the actual XMPP client is running on their server, and can maintain presence and receive messages even while your phone's connectivity is wobbly.