Apple's fixed resolutions, whilst allowing developers to ensure a tight design on all devices, kind of dig them into a hole when it comes to changing their devices' screens.
The change from the first 3 generations' 480x320 to iPhone 4's 960x640 was smart, as the aspect ratio remained the same and also because it was an integer multiplication of each dimension (x2), so there was no problem upscaling old apps.
iPhone 4S's 960x640 to 1136x640 was smart too. They changed the aspect ratio but kept one dimension the same, increasing the other. Black bars at top and bottom helped with the transition, so there was no problem displaying old apps as they were designed.
But now, I'm sure they can make a change to aspect ratio, size, or resolution in such a way that minimises the impact on developers like they have the other times. They are unlikely to want to change the aspect ratio again. Unless they lower the pixel density (going against all the Retina marketing), any increase in screen size requires an increase in resolution, but I really don't see them doubling the pixels in each dimension again. That would be 2272x1280 - surely a battery killer, and even if they go up to 6", that's 435 PPI, a bit overkill. But there'd also be problems if they applied a decimal multiplication of pixels.
So I'm quite interested to see what they do.