Are we talking about the same thing? XMPP federation? If I've got an account on jabber.org or run ejabberd on my domain, I can add you@yourGAppsDomain.com as buddy, see your presence and can message you, without caring about the server your account is local to?

Edit: See elsewhere in the thread: You are correct, I am wrong. Federation is supported for GTalk since 2006: http://googleblog.blogspot.com/2006/01/open-federation-for-g...