It’s attractive, no question, but it sure looks thick to me.
Despite appearances, at .37 inches the alleged Apple prototype is actually quite a bit thinner than the current model’s .48.
It shows what Apple knew all along, tapering the edge, and using black, gives the appearance of thinness. The first 3G model was slightly thicker than the original, so the new plastic back was designed to help hide this fact. If the next generation iPhone is indeed thinner, they can go back to a more-straightforward design, albeit at the cost of it looking thicker.