Google Voice integration with Hangouts is rolling out to some users right now. Yes, this is real. Though, for the moment, the rollout seems incomplete.
As Android Police points out, some users have confirmed Google Voice integration is active in Hangouts. There are still some kinks to iron out (Android Police speculates that some features still need to be rolled out as part of Google's typical "Update Wednesday"), but the promise to send and receive SMS and voicemail seems to be finally coming true.