Unimaginative, purely speculative concept renderings of the iPhone 4G can be found all over the web, and they're essentially the iPad scaled down, with the aluminum unibody and everything. But one technical problem that makes an aluminum-backed iPhone unlikely is that the metal would block incoming signals.
Over the weekend Engadget posted what are allegedly photos of an iPhone 4G prototype, sending the web a'Twitter. Daring Fireball's John Gruber believes the photos are the real deal, and added this interesting tidbit: He believes the back will be made out of glass, or a glass-like material like zirconia, that will be strong, scratch-resistant and most importantly, radio-transparent.
The iPhone 4G launch date is still a couple of months away, so we'll hedge our bets. But if there is a glass back, maybe the device will finally have the Mighty-Mouse-like "back-touch" feature we incorrectly hoped the iPad would have.