Google will stop XMPP support after May 15, 2014. So for those using asterisk to make Google Voice calls, it will not work anymore. Google is also pushing to replace GTalk with Hangout. It is very possible that GTalk will not work in asterisk neither. I am wondering if Asterisk developers have plan to provide support to Google Hangout in the future?
You would have to ask that on the developer list. However, I would presume anything would be done as community supported development, rather than Digium supported, so only limited effort would be available.
Is there are public protocol specification for Hangouts?