Giz posted image of what claimed to be the iPhone 4 for Verizon. "Our tipster claims that the Verizon iPhone has a "brushed aluminum back and a standard black front." He says that they will come packaged with a bumper. He also points out that there will be a product(RED) version of this phone."
In addition to that, there have been credible reports on Apple engineeres being aware of back glass problems that make the iPhone 4 design too fragile. An aluminum back makes sense, and this retro brushed styling looks like something that Jon Ive would do.