Here's my take as a designer (graphic, not industrial):
I feel this iteration is a return to class, after the cheap plastic successor of the original, classy, iPhone. Now that the chrome bezel has been ditched, the iPhone finally looks like a member of the Apple family -- specifically, the MBP, the new iMac and the iPad, all of which are squarish and based around the contrast between aluminum and glossy black, not roundish and plastic and framed with chrome. The chrome is a leftover from retired iPod models and inconsistent with *all* current offerings from Apple, and furthermore it's scratch prone and a finger grease magnet -- good riddance I say.
As for ergonomic considerations, the curved back served no such purpose anyway because you held the 3G/3GS by it's thin, sharp-ish and slippery edges. Now there will be a flat aluminum surface where those edges used to be, which provides for much better grip, and thanks to the flush and flat back, the damn thing will finally lay flat on a table top instead of rocking and spinning.