History stuff that developers can skip...
The original iPhones had a resolution of 480x320. Going to the iPhone 4 got 4 times as many pixels, but kept the same "logical" resolution, just going to half sizes for a new physical resolution of 960x640. Buttons and everything else would be the same sizes and positions, so apps didn't have to change to work. Developers could give a new image with a bigger size and @2x as part of the name, and everything magically worked to use the higher resolution image.
iPhone 5 got a little taller screen and has a logical size of 568x320, with a corresponding physical resolution of 1136x640.
The rumored 1704x960 continues Apple's retina trend and is 3 times the size of the iPhone 5's logical pixels of 568x320. Existing apps would be slightly stretched, but would all work exactly the same. I'm sure developers can have new images named @3x to get extra sharpness at that new screen size.
If 1704x960 the new size, and I'd bet a lot that it is, I'm sure Apple will do all that. Easy and straightforward, same sort of transition and image resolution updates everyone has gotten used to with Retina screens. Everything works as-is.
But what if they also add a new option developers can set to say "I want a new Logical size of 852x480"?Instead of simply being stretched to use the new bigger screen, it would give a new logical size to work with. That seems it would be a cool additional way to take advantage of the new bigger screen. Especially for things that deal with text like web browsers and especially web pages.
And has an additional advantage that an app wouldn't need 3 sized copies of the same image 1x, 2x, 3x. Just 1x and 2x.