"We're making some pretty big assumptions about usability without having actually used the device," Barnard said. "Over time, I think the iPhone 5 may completely change UI paradigms... It's certainly a lot more pixels than we're used to." Once Barnard and his team have the iPhone 5 in hand, he said, "we'll reevaluate and start thinking about the long term implications."

Not every app will be ready for the iPhone 5's larger screen right away. Those apps will run on the iPhone 5 in letterboxed mode, with black bars providing padding on the top and bottom in portrait mode, and on the sides in landscape.

That's reminiscent of the way the iPad can run iPhone apps--nestled inside the screen, but not filling it completely. Of that approach, Barnard said, "I bet it will feel good enough to be usable, but bad enough that users will notice, which will in turn encourage developers to update their apps." He added: "I doubt it will feel anywhere near as bad as iPhone apps on the iPad"--a sentiment echoed by several developers we spoke to.

Pierce added that Apple's approach--centering unoptimized apps on the iPhone 5's screen--"is hard to evaluate without holding the device. I think it's likely it will feel somewhat awkward, but is also likely the best solution." Apple could have perhaps position such apps directly against the bottom edge of the screen instead, but that would have made the unused space even larger and more prominent.