As you may know, Apple provides some case manufacturers with the size and shape of the new iPhones and iPods so they can start making the new cases before the devices come out. This doesn't allow us to know what materials Apple will use, but we can have a good idea of the shape of the future device.
Here is the photo illustration of one of the case manufacturers of the iPod Touch 4.
Even if it is hard to have a perfect view through the case, it looks like the back is rounded, unlike the one of the new iPhone, and of course, there is a camera and a flash.
This source is very reliable since it is already thanks to it that we were able to predict the camera on the iPod Mini last year.
It would not be surprising that Apple kept a different design for the iPod Touch and the iPhone, especially for the back made of glass because they have a hard time making enough of them. (that's also true for the front)