Received mine today here in Spain. 16Gb, black model, serial DNX...
The screen is different from the one in the iPhone 4. The colors look less "crisp" in the 4S, and the whites are yellowish.
I watched over 50 movies (previews) in iTunes with both phones side to side and I have to say that all of them looked much better in the 4S. The colors are more natural and there was a lot of details in dark zones that doesn't showed in the 4.
I think that the eye would adapt over time, but I have a big doubt, is this a intended change or a faulty lot of screens? Like in many other cases (antenna or proximity sensor issues) I don't expect any kind of statement from Apple acknowledging that there's a problem.
If I keep the iPhone know and in a couple of months all screens produced are more white I would end with a "faulty" iPhone than nobody would want if I resale it in the future.
Looking for some clues in the Apple website I found the video about the iPhone, "Watch the iPhone 4S video".
Go ahead and play it, then forward to 3:40. You would see the camera app and later the locked screen.
Is yellow and is exactly the screen that I have right now. I want to think that Apple used a "good" unit to make this video.
Other screenshots in the Apple website:
If you look at "Built-in Apps", then "Browse All..." then "More Built-in...", you can see the "Notes" screenshot, it had a level of yellow that is more similar to my 4S screen, but mine is less saturated.
The "Calculator" is identical (look at the results display, my 4S had the same color).
The "Reminders" app looks white in my iPhone 4 but not in the website, whick look more similar to the color in the 4S.
I find confusing the fact that all looks great, except the whites. I don't expect that this would improve over time.
More strange to me is the fact that some people had white screens (but nobody published any picture). I wonder if those people are using the full bright? If I raise the bright in the 4S you can hardly tell which one is the 4 or the 4S. Obviously that level of bright would toast the battery too fast.