Based on the size of the chamfer on the top and bottom edge of the supposed back shell, I don't think that the front glass will sit completely above the rest of the phone like it does on the iPhone 4/4S, but rather be closer to being flush with it like on the iPod touch or the iPad 2. The chamfer on the top and bottom edge of the band on the 4/4S isn't nearly as large because it isn't the edge that your had really contacts (it does, but to a much smaller extent than it contacts the edge of the glass). If this is the case, then the phone shouldn't be much thicker than what you see here or than the steel band of the iPhone 4/4S. Basically they got rid of the glass thickness. Probably able to do this if they're using the thinner Gorilla Glass 2 and the reduced thickness touch/LCD assembly.
The design is growing on me, seeing it side by side with an 4/4S it doesn't look awkwardly long to me and I don't mind the two-tone back.