The article that tipped me off was Beyond REST: Building Data Services with XMPP PubSub over on Anarchogeek. They basically proposed hacking XMPP into handling data services, as an alternative to constantly polling feeds. It's a great idea, because XMPP can handle events. As they stated, instead of a constant stream of "are we there yet," you can basically just say "let me know when we're there."
Since then, I've noticed momentum building up around the idea. xmpphp and sleekxmpp popped up on Google Code. Better yet, Gnip actually put the idea to use, by offering a 3rd party service built around XMPP.
Needless to say, I think we're going to see this or something like this gain a lot of ground in the near future (at least for high-traffic web services). It just makes more sense than constantly polling a feed.