my five cents....
...I actually like the design. Most likely, what Apple learned from the iPhone 4(S) is that an exposed antenna causes issues when people touch it despite nifty coating etc. Then, you have several choices: Glass, plastic, metal, or ceramic.
Glass is brittle and even Gorilla Glass can scratch. There were a lot of complaints and even lawsuits about the glass back. Also, glass usually needs to be thicker than metal.
Plasic is out of question for Apple. They can't talk it down when competitors use it (because of its obvious disadvantages) and then use it.
Metal (most likely steel or aluminum) is a preferred design choice by Apple but it has the problem to act as a
Faraday Cage meaning not letting the antenna do its job. Hence, Apple probably chose to put some different material on the top and bottom part.
Ceramics would be my choice. It is rather expensive to mass-produce in comparison to glass because it has to be backed hotter than liquidation of glass and undergoes a controlled shrink etc. It is not as brittle and virtually scratch resistant. You would need a diamond to scratch it, basically. Given that aluminum scratches easily and even steel scratches as well, the corners made of Ceramics would be nice.
So, my hopes are on ceramics as the corner pieces.
