Ok, the iPhone5 has a taller screen. It runs older apps fine despite the fact that you will have two ugly black bars in the top and bottom of the screen.
But what about the apps creted with the iPhone5 screen in mind? How will they run on the older iPhones? Will developers have to create 2 versions of each app?
I'm curious about this too. For this reason, I actually held off on getting the iPad 3 (I was already on the fence). If the next iPad uses the new resolution, I rather wait for it if it'll use the new dimension.
However, it's now about writing a completely new app for a different resolution, it's more about supporting the new resolutions. Think of it like going from the 3GS to the 4, new resolution meant adding support for the new feature to really shine.