The major difference for iOS 6 on the iPhone 5 is the change in aspect ration from 2:3 to 16:9, and in pixel count from 960x640 to 1136x640. That means everyone from Apple to App Store developers have an extra 176x640 pixels to play with.

For Apple, 16:9 allows for an extra row of icons on the Home screen and in folders, an extra list item in Mail or in Music, and a bigger display area in Maps and FaceTime. Since, unlike the iPad, the iPhone is based on a single-column interface, the lack of additional width is seldom if ever an issue in portrait mode. Curiously, Apple isn't vertically centering album art on the Lock screen with the iPhone. Hopefully that's a bug scheduled for imminent squash-age.