Core77 had a chat with Apple's Jonathan Ive about the iPhone 4 and the materials that it's made out of, and Ive says that the process is very holistic; Apple has really gone from start to finish with the types of metal and glass that make up the latest iPhone, and they've worked at every step of the process to try to make them better designed.
He says that the glass on the front and back is "scratch-resistant aluminosilicate glass," and the metal around the edges is one full band of stainless steel. (The "seams" are just cosmetic -- it's all one piece, apparently.) "That understanding, that preoccupation with the materials and processes," Ive says, "is essential to the way we work."
Ive does have his usual ethereal lightness about how hard design is to grasp. "You cannot disconnect the form from the material -- the material informs the form," he says. If nothing else, though, the interview definitely shows just how obsessive he and Apple are about designing and manufacturing these devices.
iPhones become commonplace so quickly after launch that you tend to forget all of the work and thought that has gone into every single feature of their hardware. Apple isn't really doing anything magical here; it's just sitting down and grinding out exactly what materials work best in which ways in order to make a really beautiful and functional object.