Apple doesn't do "S" ... Wait... Yes they do
Of all the surprises following the iPhone 4S announcement, the greatest seemed to be directed at the name and the fact the phone did not receive a major physical redesign.
Why, I can't imagine. Discounting the original iPhone, Apple has released the iPhone 3G, iPhone 3GS, and iPhone 4. There was some possibility all along that a new product moving forward would evolve into external redesign -> internal upgrade -> repeat. Yet rather than acknowledge this, popular rumors focused on the creation of a new iPhone 5. Which would be...slimmer? Stronger? Who knows.
Indeed, the "who knows" makes one wonder why a physical redesign of the set was craved at all. The original iPhone 4 body shell remains among the most attractive, and thinnest, designs on the market today. Heck, even the iPhone 3GS still looks nice, which is probably why the 8 GB model is still in production.
There was no need for Apple to change what isn't broken. So they didn't. It's really as simple as that. Even a company as design driven as this does not need to, and may in fact be damaged by, a design cycle that entirely replaces a key product in about a year and a half.
The good news is that the iPhone 4S is still durable and beautiful because, well, it's an iPhone 4.