Update: Dori Storbeck now says Google has "nothing to announce at this time."
Original: If you were wondering why the new Google Hangouts app asked you to confirm your phone number, this might be the reason. Googler Dori Storbeck, Community Manager for Google+ Hangouts and Chat, has confirmed in a Google+ post that SMS integration will eventually be headed to the new messaging app.
Responding to a question about the unification of Hangouts and SMS, Storbeck said "SMS integration is coming soon - it is one of our most requested features!" There's no information on when this feature might arrive, nor what the implications might be for built-in SMS apps. If nothing else, it's another baby step towards the true unification of messaging that Hangouts aims to deliver.
Source: +Dori Storbeck