If rumors are true and Apple announces a 4-inch, 16:9 iPhone 5 on September 12, it'll be interesting to see how it handles existing content and apps, including games. A couple of months ago, I spent some time making a ton of mock-ups to show how various interface elements might work. Now, MacRumors has put their take on it together for the video above.
Some of the answers are obvious. Content Apple controls, like the Videos app, will just work at 16:9, adjusting letter-boxing as appropriate. Third party apps already built for the current 3:2 screen, however, will likely get letter-boxed (vertically) or pillar-boxed (horizontally).
Updates to take advantage of the full screen will likely be simpler and faster for apps that use UITableView and UICollectView for lists and grids, and OpenGL for games, and likely more complicated and slower for apps that use a ton of custom, specifically sized and positioned interface elements. (AutoLayout in iOS 6 might help somewhat as well, if it's used.)
While Apple doubled the screen density of the iPhone display when it went from the standard iPhone 3GS to the @2X Retina display of the iPhone 4, moving to 16:9 would be the first aspect ratio change. Since the density won't change -- there'll instead be 1136x640 instead of 960x640 -- individual elements likely won't have to be redrawn the way they had to be in 2010, but the interface as a whole will still need some attention, especially by pixel-perfect designers.
Apple typically has at least a few, properly NDA'd developers up on stage at product announcements to show off samples of how new technologies look and work, so hopefully we'll see for ourselves come September 12.