The Complicated and Simple Reasons Why the 4.6-Inch iPhone Rumor is Bunk
Expanded from a comment in the other thread.
Reuters has picked up a story about how there supposedly will be a 4.6-inch iPhone coming out this year. It attributes the rumor to a South Korean publication called the Maeli Business Newspaper, which itself heard this from an "unnamed industry source". Nothing like third-hand information to get everyone going on a Thursday.
The kernel of truth to this that makes it worth considering is that the South Korea-based Samsung is the supplier for the third-gen iPad Retina displays. It's a major supplier of components to Apple, so there are, in fact, some people in South Korea who are familiar with Apple's future roadmap.
There are two big reasons why I am very skeptical of this rumor. One is complicated, while the other is simple.
Let's start with the complicated one. The iPhone screen has always used a 3:2 aspect ratio, first at 480x320 and then at 960x640. The reason why Apple used the same ratio when going to a Retina display is that it wouldn't require developers to re-code their apps. Everything would just scale up 2X, and that would do nicely until developers double the size of all of their raster image resources.
Using a different aspect ratio for the screen would have added a lot of headaches for developers as the layout would have to be tweaked. That is part of the fragmentation problem with Android; it's not just about a wide range of OS versions in the wild but also different screen sizes. The screen dimensions matter greatly when all apps are full-screen apps. Apple took the same tack with the third-gen iPad's Retina display, as it is double the resolution of the first two iPads' displays.
When Apple introduced the "Retina display" term for the iPhone 4, it claimed that for the distance that phones are typically held from the eye, the screen must have 300ppi to qualify for the "Retina" title. A 4.6-inch screen at 960x640 computes to about 251ppi. In order for this mythical, monstrous iPhone to keep the Retina designation and maintain the same aspect ratio as previous models, the resolution would have to be doubled again to 1920x1280. That comes out to a ridiculous 502ppi, clearing the "retina" bar with ease.
While the A5X is certainly capable of driving that many pixels, as the third-gen iPad has more than that, I don't know if it could do it in a phone form factor without draining the battery too quickly. Let's imagine that it could though, granting that the larger frame of the phone would give more room for a sufficiently large battery.
App developers would have to put three different sizes of their images in their packages, one for the iPhone 3GS's 480x320 screen, one for the iPhone 4/4S's 960x640 resolution, and yet another for the new 1920x1280 screen. Not only would that be a pain for developers, but each app would take up a lot more room (especially photo-heavy apps). The 3GS and 4 models still on sale only have 8 GB of flash memory on them, and the 4S starts at 16 GB. Storage space is a significant constraint, and forcing apps to have three different sizes of images would be untenable.
So that's the complicated reason. The simple one is that a 4.6-inch phone is simply too big for most people.
The iPhone's 3.5-inch screen wasn't chosen at random. It's roughly the biggest screen you can have where everything can be operated by a single, normal-sized adult hand. It's unlikely that anything will be out of your thumb's reach while holding an iPhone. On 4-inch and larger phones, it becomes difficult to impossible to operate one-handed, in particular being able to access both toolbars at the bottom of apps and the notification drawer at the top of the screen. That is the kind of detail that Apple considers when building these things.
I'll never say never about a larger iPhone in the future, but jumping to a 4.6-inch display this year isn't likely. At the very least, such a jump would probably require that most of the non-Retina display and 8GB iPhone models be cycled out of use, and that won't happen for at least two years following 2012's new iPhone announcement.
For what it's worth, the largest a 960x640 screen can go while still being above 300dpi is 3.8 inches. That might still be small enough to operate in one hand, but it would feel like change for change's sake. Change for change's sake is not the sort of business Apple is in.