I had experimenting with XMPP on my long term list of things to do. I took a 90 minute break from work this afternoon to set up a playground: OpenFire XMPP server and the Ruby XMPP4r client library. Setting up the OpenFire service on one of my leased servers was easy - a very good administration web application and in general an easy install.
I had more problems with XMPP4r but setting Jabber::debug = true helped. I installed the easier to use wrapper library xmpp4r-simple but decided that its API was probably too limited (long term), so I might as well get used to XMPP4r.
I also grabbed the Common Lisp XMPP client cl-xmpp but experimenting with Ruby clients is probably easier. The OpenFire developers also supply a Java client library (Smack) that is on my list of things to try.
I think that XMPP may be a good "push" technology for distributed knowledge sharing systems (an interest of mine). XMPP has a lot going for it: a good security model, straight forward bi-directional communication between any two connected clients, and a publish/subscribe capability like the Java Message System (JMS). The Comet architecture (uses HTTP and JSON, instead of socket connections and XML) looks interesting but XMPP seems to have a head start and I don't think that I need to learn both technologies (yet).