I really hate the greasy look and feeling of the glossy 3G back after you've used it for more than 30 seconds, so I'd really like a matte backing. Anyway, the images are supposedly from China, etched with a brand new model number.
A few reasons to be skeptical: Crappy reception with metal is one of the reasons why Apple went to plastic in the first place (if there's really metal in it), and it's really early for a product likely on a June release cycle to already be exposed. And all the other usual possible fakery issues with Apple leaks.
Why it's also at least partly believable: iPod Observer leaked the iPhone 3G's black glossy back months ahead of time last year (though not quite this early in the cycle).