iPhone screens have always had a 640-by-960-pixel resolution and a 3:2 aspect ratio. Indications are the iPhone 5 will have a 640-by-1136 resolution, which is very close to a 16:9 ratio, which is a common one in the industry. 9TO5Mac says it has heard that Apple will use the extra space for an extra row of buttons on the home screen.
Indeed, apps that use standard controls will likely do the same thing: just use the extra space to display more information. The controls themselves will remain the same size. But what about apps--games like Angry Birds--that paint on a canvas with what they thought was a fixed size?
Those apps, written for the current iPhone, will have dead space when run on an iPhone 5, probably at both the top and bottom in portrait mode. So developers will have to make a separate version of the app for the new iPhone, but maintain the old one for earlier iPhones. They could deliver this either as two apps in the store or as a "fat binary" that installs whatever is appropriate on the local device, or the app could just look crappy on one version or the other as the OS crops or scales it. iMore's Rene Ritchie had an excellent discussion of the problems and possibilities back in May.
This stuff isn't exactly new to mobile developers; they have had to deal with multiple resolutions and aspect ratios on Android for years, but as iMore says, it's not a very Apple-esque solution. Nevertheless, something's got to give with the new screen dimensions and this seems the most likely to me. Another iMore story from the same time frame raised some more possibilities for what they could do with the extra space, such as static space for widgets, but I have to think this would have been leaked by now, especially since iOS 6 is in beta.
Extra-large screens are at least 960dp x 720dp. (Dp stands for density-independent pixel.)
Large screens are at least 640dp x 480dp.
Normal screens are at least 470dp x 320dp.
Small screens are at least 426dp x 320dp.
Windows Phone 8 will support a smaller variety: three resolutions and two aspect ratios, and it will put an emphasis on developers using vector graphics so that apps will be more display independent.
Of course, display independence is a good idea on any platform, but Apple might have caused developers to ignore this best practice. It's easy to see that when the iPhone 5 comes out that a whole lot of apps will need to be updated, not because they don't work, but because they don't work well. They will look bad, and that's about as un-Apple-esque as it gets.
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