Honestly I think the matt back looks kind of ugly. I think a nice black bezel and a reformed aluminum body would be better. I work for Apple as a sales rep and in many discussions with my bosses, the big thing that we have been pushing for the laptops is the unibody design, and how 'green' they are. If I had to guess, I would say that the new iPhone will follow a form factor closer to that of the macbooks. A nice piece of metal on the back. No more plastic. But that is just my guess.
The mock up that was posted a few days ago "Hello, Again". That is the mockup that I would put my money on.