http://ralphm.net/blog/2006/01/17/gtalk_s2s
And in fact fanf@jabber.org and tony.finch@gmail.com can talk to each other quite happily.
However, when my dotat.at jabberd2 talks to Google, it completes dialback, then it responds to my first message or presence stanza with <stream:error> <unsupported-stanza-type xmlns=“urn:ietf:params:xml:ns:xmpp-streams”/> </stream:error>