With infrastructure becoming as big an issue as it has become and the traffic increases we've seen, I would think that it may help slightly. The major effect will be a push westward with new homes and developments down that corridor.
I hope that helps!
Although the expansion may not "increase" values per say, it may increase desirability and demand therefore shortening the number of days a home in the area sits on the market (meaning better sales price for the sellers). However if the market turns around into a sellers market again, new construction may become more enticing than purchasing an existing single family home.
I just saw a report from the builders association showing Carver County to be the fastest growing metro county in the next ten years. Being able to get into town quicker will be a positive for property values in the area once 312 is complete. However with increased gas prices the outlaying areas are under price pressure right now.