Doubt it is the case
I still have doubts about this being the back cover. Here's my reason why:
Pretend the iPhone 4 design had never come out and you're seeing the iPhone 4 design for the first time along with this proposed iPhone 5 design. Which one would you choose? I think most people would go with the iPhone 4 design. Different isn't necessarily better. I remember what Jony I've once said, "We don't change something unless we can make it fundamentally better." This proposed iPhone 5 design is not a change for the better. It is a change for the sake of change and is very un-Apple like in my opinion. As long as I have known Apple products, they have always tried to keep their products sleek and simple. This doesn't seem to follow that with the aluminum, glass, etc.
I personally think they could go another round with this design since they have just increased the screen size, so it will be actually different. Then again, it might pose a problem because the 5s, if it is called that, will be the same design if we are to follow Apple's history. I don't think people will be too happy about having a pretty similar design for 4 years on a mobile device.
I will certainly buy the iPhone 5 even if it is this design. However, I have a gut feeling that this isn't the real case. The only real plus is the unibody to me.