Apparently, the part comes from a Chinese supplier who, according to iFixyouri, is reliable. But not even they know if it is for an actual product or a prototype. This more or less lines up with past rumours of a metal-backed iPhone and a smaller dock connector, but then again, you could probably find rumours to back any combination of features you'd ever think of.
Also worth noting is that the sides of the phone are part of the back plate, which would essentially create a box for the screen and guts to sit in. And we don't want to jump the gun on this, but if that subtle two-tone look ended up in the final product, we wouldn't be opposed. [9to5mac]
UPDATE: MacRumors dug up the same case design in white, which was sent to them by ubreakifix. The font where it says "iPhone" looks weird, but the design jives with everything else we've seen. And assuming the image isn't artificially stretched out, it looks like this case would be housing a larger screen.
UPDATE 2: 9to5mac has better pics of the rumoured new case in black and white. They also have shots of a front panel, which has the front-facing cam aligned directly above the earpiece, but otherwise looks similar to the iPhone 4/4S. And yep, that black and charcoal combination looks pretty good.