If your application is iOS6 only, the easiest way to make views that layout correctly in both 3.5-inch and 4-inch iPhone 5 designs is to use AutoLayout in your XIBs to make them adapt their size automatically (See the WWDC'2012 video sessions about that). If your application must support versions prior to iOS6, then you can still use AutoResizingMasks on your views to make them resize themselves to adapt to both 3.5 and 4 inch screens.
Then for your application to support the 4-inch display and take the whole screen when launched on an iPhone 5, simply add a "Default-568h@2x.png" launch image to your project.
The simple presence of this launch image will make your application launch full-screen on iPhone 5 instead of having black bands at the top and bottom.
