Back when the iPhone 4 was launched, apart from issues with the “death grip”, there were complaints about screen discoloration. This was apparently due to the swift shipping of the device, which resulted in the glue used to hold the screen not drying properly, thus giving off a yellowish discoloration. Based on user feedback in the Apple forum, this problem seems to affect users of the black iPhone 4S as opposed to the white version.
However the good news is that if this is indeed another issue with the glue not being dried properly, iPhone 4S owners can expect their screens to return to a nice, bright and crisp display in a matter of a week or two. At the moment it has been reported that Apple is allowing the exchange of devices for owners who really insist on it.