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.
This seems to be a server-side switch and not everyone is seeing it immediately. Those who are don't necessarily have all functionality working yet. If you see the update, let us know in the discussions below to help confirm what's working and what isn't.
Google Voice Integration Is Currently Rolling Out In Hangouts | Android Police