Hangouts already features Google Voice Integration
As if we didn't need another reason to believe Hangouts will integrate or and/or replace Google Voice here is some proof.
Anyone with a Google Voice account should be able to do this. I run the Canary build of Chrome on my desktop with the Hangouts extension installed. When someone calls me via my Google Voice number (which is my primary number) Hangouts notifies of the call and then allows you to even answer through the computer regardless of if the caller is on a smartphone, uses hangouts, or calls from a landline.
I tested this with my phones T-mobile number and this does NOT work, so it is a back-end integration with Google Voice.
The mobile app does not demonstrate this sort of action.G
Here are some screenshots: