With the introduction of the XMPP service to App Engine, it’s now possible to write an App Engine app that communicates with users – or even other applications – over XMPP, otherwise known as Jabber or Google Talk. In this article, we’re going to walk through an example that covers all the basic functionality of the XMPP API.
http://code.google.com/appengine/articles/using_xmpp.html