Google has flipped the switch on their mobile boarding passes in Google Now. Starting today if your itinerary is linked to your Google account, Google Now will prepare your mobile boarding pass and you can use a QR code to board your flight.
Google Now's boarding passes only work with a few select airlines at this point, but obviously more will follow in the future.
I've been using the American Airlines app to board flights for the last couple of years and it's still faster to just whip out the paper pass. Sometimes the scanners don't work, or your phone goes to sleep and it takes too long to bring up the app. I'm sure someday we will all simply use our smartphones but for now it's more of a novelty.