XMPP, Publish-Subscribe, PEP and User Tune

[Starting this again, now I understand better how it works and have actually updated Psi to a version that receives User Tune notifications...]

BBC Radio Labs has a new prototype which broadcasts metadata for tracks played on the radio over XMPP.

If it was using standard Publish-Subscribe (PubSub) nodes (i.e. if it was broadcasting generic events), it would look like this:

But it's actually using the User Tune protocol, which is based on PEP, a special case of PubSub where each user automatically becomes a PubSub node, and all the clients in their roster ("buddies") are automatically subscribed to their notifications:

Clients (such as a "Now Playing" page) subscribe to a specific user - radio1, for example - by adding them to their roster and being authorised to receive presence notifications; whenever that user's status changes the message gets broadcast to all the subscribers.

Here's an example message:

<message from="radio1@hug.hellomatty.com" to="alf@hug.hellomatty.com/Psi">
  <event xmlns="http://jabber.org/protocol/pubsub#event">
    <items node="http://jabber.org/protocol/tune">
      <item id="1203447868">
        <tune xmlns="http://jabber.org/protocol/tune">
          <artist>Panic! At The Disco</artist>
          <title>Nine In The Afternoon</title>
        </tune>
      </item>
    </items>
  </event>
</message>