We announced a new communications product, Hangouts, in May 2013. Hangouts will replace Google Talk and does not support XMPP.
If you are building your own XMPP service, refer to the XMPP specs. If you are using a commercial or open-source jabber server such as ejabberd, then federation should just work. One important thing to note is that we use dial-back for authentication. If you have any other questions, feel free to post to the Google Talk Interoperability Google Group.