There is an open discussion on Apple’s discussion forum where iPhone 4S owners are reporting problems they are having with their new phones display. The majority of complaints are about how the display has a yellow tint which is especially clear when you place the iPhone 4S next to an iPhone 4. There are other lesser complaints about color washouts and contrast in the same discussion thread.
This looks like a problem that plagued the iPhone 4 when it was initially released. That problem was due to the iPhone 4 shipping so quickly after it was built that screens continued to show signs of a yellow adhesive used during manufacturing. Eventually the yellow residue from the adhesive disappeared leaving a normal looking display.
If you have one of these phones then what I would do if I were you is to call AppleCare and log a case. Previously people suggested that you wait one or two weeks for the residue to clear. I don’t think that is unreasonable considering the iPhone 4S has a one year warranty.
My iPhone 4S 32GB Black AT&T model seems just fine to me. I’ve enjoyed using it and speaking to Siri has been a lot of fun..