In the iOS simulator that Apple provide for iOS developers there is an option to run your app on a simulated "iPhone (Retina)". I know the iPhone 4 has a "retina" display but look at the picture below. It has a very small "home" button, barely visible just above the grey line at the bottom. If the grey line delineates the iPhone it's not the same shape as the iPhone 4 and  it means almost all of the new iPhone is the screen. This is certainly credible but it beats me why they would show it to developers in advance of the launch.