It's plastic. A very strong kind of plastic. I'm no material engineer or anything but everyone knows there's a huge range of plastics so using the generic 'plastic' term to describe the back of the iPhone 3G isn't wrong.

Check out WWDC 08 keynote 1:26:13
"It's beautiful. This is what it looks like. It's even thinner at the edges, it's really beautiful, it's got a full plastic back, it's really nice".

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Added on August 10, 2008, 7:24 pmedit to add: I've played around with the iPhone 3G quite a bit, and stringfellow's right about the back not feeling like a typical plastic mobile phone (like Nokia and Sony Ericsson phones). However, at the end of the day, it's still plastic

This post has been edited by lailaikatong: Aug 10 2008, 07:24 PM