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Here's a good write up http://thetechblock.com/the-unibody-iphone. It's for WiFi/GPS.

The two glass sections contain various antennas which have signals that cannot pass through the aluminum well.

Some have said it may be to improved antenna/wireless comms performance

Just a guess, but could it be to alleviate previous reception issues?

Interesting point.

I fully agree with the OP here, the back looks bad. I don't get it. The all-glass back on the 4S was really nice. Had a certain allure and design sense to it.

Apple this time around:

Tim Cook: "Hey Jony, let's make the back uglier by anodizing the steel and let's see how many we can sell before we release the 5S next year."

Jony Ive: "Alright Tim, I bet we'll sell more than the 4S with an uglier case, you're on"