I still don't like letting the screen dictate the image I am sending. I'd rather something at the OS level like "they're on a crappy connection, send them a 1x" and "oh sweet wifi, let's get nuts" or "eak, battery low, grab the 1x"
The whole 2x image thing bothers me a bit anyway. One of the first things they tell you to do to make your page faster is to not scale images but with 2x images it's ok. So why wouldn't you just serve everything 2x and avoid a headache of multiple images.