They wouldn't put the wrong specs on there. And Steve wouldn't have said "Same great 3.5 inch screen"
The difference in images may come from the fact that the form factor of it is different. More tapered on the back. If you have an iPhone now, look at how the back panel starts to curve back around to the front before the chrome bezel. If you look at the side images from the page you linked to on the 3G. The back goes straight up to the chrome and doesn't curve before it. That would make up for the approximately 2mm of extra space you are noticing on the front.