Another day, another parts supplier with a video of the iPhone 5 back. This video has a good explanation of the differences. The best shot is the one below which shows the thinness of the new iPhone 5 back. Remember that the screen will still need to attacted, adding a few mm, but we can tell it's thinner than the iPhone 4S.
The iPhone 4S is made of a three parts: metal mid-plate, a back panel, and a front panel. The iPhone 5 design incorporates the back panel into the mid-plate, which decreses the thickness of the phone by the thickness of the back panel. In other words, looking at the iPhone 4S, the 5 will be less thick by one width of the black glass panels. It may not sound like a lot, but in your hand it will feel like a lot.
Here's the full video: