Having been to 2 Apple stores and one AT$T store today, specifically to compare screens for an unrelated issue, I have come to the conclusion that the iPhone 4S screens are among the worst in terms of consistency of any iPhone release, especially when compared to the iPhone 4, which I found to be the most consistent. As a professional photo retoucher, I feel somewhat qualified to pass judgement.
The screens I saw varied considerably, from neutral and contrasty, like the iPhone 4, to flat and yellowish, like the 3G was for many. And since I was investigating issues with viewing the screen through polarized sunglasses (a can of worms on its own), I can add that doing so presents a wildy varied experience
some were deep blue, some were magenta, some green.
Oddly, all but the Sprint models showed discoloration when seen thru polarized sunglasses. The Sprint phones looked good. This says to me that the coatings, the glass, or even the LCD itself are all over the place in terms of tolerances, manufacturing methods, and/or suppliers.