The rumors have been hot and heavy that the iPhone 5 will have a 4-inch screen (compared to the current 3.5 inches). And the big question was how Apple would accomplish that, with the latest round of rumors saying the current prototypes simply add pixels to the height, while keeping the width the same. The result is that in landscape mode you end up with a 16:9 aspect ratio, just like the widescreen shape of an HDTV. So now the speculation is how that space will be used. Again, Rene Ritchie of iMore has an excellent analysis of the options -- and what it all means. He includes very helpful mockups of these options, giving you a clear picture of how a wideiscreen will alter your user experience.
He considers, and pretty much dismisses, the possibility that the added space would be used for static system content. For example, the app doc might simply fill the extra space and be continuously present, regardless of what app you're using. More likely, the extra space will simply be available to developers to redesign their apps to take advantage of the additional screen space. One of his more interesting mockups shows the virtual keyboard with an additional row of numeric keys.
Most of his mockups, and the others appearing, are presented in vertical mode, where the additional space is more noticeable. But I'm guessing that if Apple goes the 16:9 route, they'll make a big deal about the widescreen format for video playback.
iMore, of course, cautions that we're still in the early stages of prototyping, and Apple is likely leaving all options open. Who knows, they could end up deciding to continue with the 3.5-inch screen. But a 4-inch screen is seeming increasingly likely.