They actually explicitly dismissed XMPP (see the comment on this video from them: http://vimeo.com/13026173). They were also approached by the onesocialweb.org team to work together and also dismissed the invite.

It seems however that they plan to align with the ostatus.org protocol, but so far no documentation of their architecture and protocol has been delivered. In Ostatus, the server-server messaging is HTTP based and using the PubSubHubBub and Salmon protocols.