One of my favorite thing about the iOS world is how many AAA industry veterans are returning to their roots of indie development, potentially bringing decades of experience with them. This is the case with Glenn Corpes, whose 20+ years in video games has had his hands in Populous, Dungeon Keeper, and a ton of other great games. On the App Store, he's released Ground Effect [$2.99] and is hard at work on Topia: Age of Creation.
Anyway, in a recent interview with PocketGamer.biz Glenn explains that even though the iPhone 4S looks identical to the iPhone 4 on the outside, the internals represent an even bigger leap than the one between the iPhone 3G and iPhone 3GS. Corpes calls A4 devices "irritating," as they had tons of potential utilizing shaders, but were ultimately too slow at processing the actual pixels. Things were only made worst by the Retina Display, as App Store customers quickly became disinterested in anything that didn't push all 614,400 pixels.
He explains that some of the cool shaders we've seen in Infinity Blade 2 could be done at a lower resolution, but you just can't release games like that. A5 devices, on the other hand, are powerful enough to use shaders that would have only been useful on PC's and current-generation consoles before. While fully utilizing the 3GS required code rewrites, developers can take advantage of the 4S by simply using better shaders. Even more spare horsepower can be utilized with anti-aliasing, post processing effects, and other things A4 devices weren't capable of.
I imagine just like the 3GS, it'll take some time for developers to actually start taking advantage of all the new capabilities iPhone 4S. I can't wait.