That’s no illusion-it’s an Office Communicator user communicating with a Gmail user. If you can’t wait to try this, you have come to the right place. This article assumes that you’ve an environment running Office Communications Server 2007 R2 complete with an Edge Server that’s configured to allow internal users to federate with external domains. You’ll have to request a certificate for your XMPP Gateway. For Gmail to locate your XMPP Gateway, you’ll have to create SRV and A records on your public facing DNS. To install the XMPP Gateway, you must’ve a separate server running Windows Server 2008 R2.
Full Article: OCS blog