I don't think there's any question that there are different screens. I've had experience using three iPhones - one 4 and two 4s's - and they have all been different. My 4s from this January is noticeably warmer than my girlfriend's 4 from last September, but the difference isn't drastic enough to me to warrant a replacement.
Because I was having other issues I did get a replacement 4s last month, and it had an extremely blue/cool screen, and also looked brighter than both my original 4s and my girlfriend's 4. After using that phone for a few days it made both of the other phones look warmer and the whites not as "white". However, I felt the colors were almost too unnatural and it had a really uneven screen. Because I didn't want to play the phone lottery I decided to keep my original device.
After being on my original device for a month again now it seems fine, and I think it really comes down to our perception and what our eyes are used to. I know my phone has a warmer tint to it, but in daily use the whites seem just fine and the screen looks fantastic. It's only when I compare it to another phone can I recognize any difference, and I've yet to see a 4s that was as crazily blue as the replacement I had.
It's not like I'm using my phone side by side with another phone every day, and since my eyes are used to my screen I think I'm okay with it. If you're used to a blue screen I can understand why one would be upset, though, and there is clearly a lot of variation in screen temperature/gamma.