physical mockups of the device, which point towards the device being taller, but a bit thinner than its predecessors, so the extra row of apps makes sense. Apparently, someone was playing around in Xcode and found that by setting the resolution to 640 x 1136 or higher, the default setup of the home screen changes from 4 rows of icons to 5. This only happened with the iOS 6 emulator though, with the iOS 5.1 emulator the default was 4 rows of apps with extra space in between.
So, that seems to point to the next iPhone having at least a 640 x 1136 resolution, but we'd expect Apple to hit the 720 pixel mark on the width in order to be able to market the device as HD. Still, it's good to know that even though we still haven't seen any evidence of a widget/live tile competitor from Apple, the company is going to put even more app icons on the screen.