John Gruber explaining how Apple could keep a 4″ iPhone under wraps while still getting developers ready for the larger screen before hand:

If Apple introduced something like this in iOS 6, they could encourage iOS developers to adopt the recommended APIs to be responsive to changes in available vertical screen space. For now, they could pitch this in the context of shrinking screen space in response to on-screen notification banners, but, come October, apps that do the right thing would automatically be responsive to, say, a new device with 176 more pixels.

That would be clever and answers one of the biggest doubts that I have/had about the rumored ‘tall-skinny’ iPhone.