iOS 5.1, which is on the right, only shows four app rows when run in the same iOS simulator app. That's telling, since there would be little reason to add the support for an additional row if the screen was not going to get bigger. 9to5mac also reports that the simulator will not display the five rows at any resolution other than 640x1136, suggesting that Apple's not just trying to squish as many apps into the current screensize as it can. Other higher-than-current-iPhone resolutions scale iOS 6 to the iPad's home screen.
This is just the latest evidence that we'll see a 4-inch iPhone. There is a phalanx of iPhone rumors cramming itself down our throats, and almost all of them point to a taller, 4-inch model with the aforementioned 640x1136 resolution.
So! Looks like big iPhones come September. That or some extra real estate on your favorite iOS simulator's home screen, at least. [9to5mac]