According to the nonprofit think tank Digital Society, former ZDNet technical director George Ou used a series of pictures and rendering to extablish that Apple used images to fake a pixel density improvement twice as good as actual. Although Retina Display will deliver twice the resolution (now at 326 pixel per inch vs 163 pixels per inch before), the images used in the advertisement above represents an approximately 4X improvement as seen in the illustration below:
People can always argue about good advertising, but just be sure advertise accurately, else put a disclaimer. Be honest. That’s my take.