We announced a new communications product, Hangouts, in May 2013. Hangouts will replace Google Talk and does not support XMPP.
Google Talk uses extensions to XMPP for voice signaling and peer-to-peer communication. Source code and documentation for these extensions is now available. In addition, these extensions are in the process of being reviewed by the XMPP standards body as official enhancements (known as XEPs) to the standard. Note that the source code for Google Talk's current implementation of these extensions varies slightly from the proposed specs. Upon ratification of the specs, Google Talk (and the source code) will be updated to be in full compliance. In the future, we plan to support SIP signaling as well.