IIRC Google does support federation. At least with Google Apps for Domains, you can add some records to your DNS that enable federation for your domain.
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?
