XMPP (eXtensible Messsaging and Presence Protocol) is the open standard for IM and presence, often referenced as Jabber. XMPP provides a specification for client access to servers and for server-to-server communication.
For example, if I am connected to the Isode.com XMPP server as Steve.Kille@isode.com, and my buddy Nancy Cox is connected to the Ferris.com XMPP server as Nancy.Cox@ferris.com, the servers can discover each other and exchange presence information and messages.
This server-to-server capability enables general interconnection between all those using XMPP servers, and provides universal IM connectivity in the same way as email. This will give an IM world to the big proprietary servers, and a distributed XMPP network.
Servers using protocols other than XMPP for client-to-server communication can still use XMPP for server-to-server communication. This provides a framework for the big proprietary IM servers to interconnect in a worldwide, open-standards IM network.
Whether commercial considerations will ever allow this to happen is, however, a different matter. Proprietary IM network operators such as Yahoo and AOL monetize their networks and won’t interconnect for purely altruistic reasons.