The original iPhone, the original Nano and of the HDD iPods have flat backs.
C.
You're right they did have flat backs. I used to own a 5th gen iPod. But it was still rounded at the sides

Flat front, tapered sides, flat back. In fact, this was Apple's signature design. 1st gen iPod Touches and iPhones followed this philosophy. Then they started loosing the flat back with the iPhone 3G as the tapering got a little more extreme until the whole back became curved. The iTouch 3G followed suit. Thicker components, thicker back and they taper the edges to make it look thin. It's probably a design trick, you may or may not be right.
But it is the rounded back or rounded tapered edges that gave the iProducts their character. I always fancy them for being round pods that's been cleaved in half. That rounded back/edge gave these products an organic feel that the flat back of the iPhone prototype simply does not capture. Even the not so rounded 5th gen iPods and 1st gen iPhones have this quality simply from their tapered and rounded edges.
Loosing that curve, the new design looks more generic. I think this is what people mean when they accuse it of very "un-Apple." I do appreciate the aluminum frame all around the side and the glass on black face because it adds design cohesion by comparing the phone to a small iMac. However the illusion breaks when we see that the back is not even at least one bit rounded just like on an actual iMac.
The fact that the iMac has a pregnant back may be an engineering necessity, but it adds character to the machine, like the black bezel and edge to edge glass. Likewise, the fact that the iPhone prototype has a flat back and non tapered edges may also be an engineering necessity, but I just wished the iPhone prototype could have somehow found a way to keep a slightly rounded back to make the iMac reference a little more blunt. Psychologically, you would be reminded of the power of a full fledge computer, like an iMac, in the palm of your hand. And personally, it would have been a cool feeling to pretend I'm holding a shrunken iMac.