To expand on this: XMPP is the opposite of a walled-garden. I've used iChat with Google Talk, I've used it with an internal Jabber server, and the 'normal' iChat is AOL IM.

Even the Bonjour chat is open - just XMPP with ZeroConf discovery.

To say that iChat is 'totally walled garden' betrays a deep lack of understanding of the software being discussed.