You can now Instant Message (IM) me on my Jabber.org account rick.parsons@jabber.org from the Google Talk service.
The Google service has been running for some time and uses the same XMPP protocol, but would only communicate with other Talk users, but now they have federated with any Jabber server so can accept interconnections.
Similarly vice-versa, users of Exodus, PSI and other Jabber clients can now message people on Google Talk.
Not voice though; that is an extension built by Google which hasn’t made the standard yet, nor any of the other clients.