Here is my photoshopped mock-up of how the iPhone 5 might look. In my mock-up, the screen is physically a little wider and quite a lot taller versus the iPhone 4S. The external dimensions of the phone are identical to the iPhone 4S.
The resolution of the display would be 640 x 1040 pixels (iPhone 4S: 640x960 pixels). The 80 extra pixels on the vertical axis will allow for more content on screen -- but is probably not quite enough for another row of icons. I chose this resolution as it seemed to fit nicely in the available space.
In order to maintain an equal sized bezel above and below the display, the home button had to become slightly smaller. I changed it to an oval shape.