Following on from this weeks release of an XMPP connector for Office Communications Server 2007 R2 (how do Microsoft come up with those catchy names eh?), the OCS team have posted some detailed information on the configuration needed to enable communications between OCS, Jabber and Gmail users.
Information on configuring the gateway for Jabber can be found here.
Info on the setup for connectivity to Google Gmail is here.
(Topology images from the OCS Team Blog)