Hello,
Just to share my experience.
Went on apple store friday, (the 25th, it was the sold and looot of people too ><), for an other exchange.
I got my third iphone 4s, it's still a C39 serial, but, and here there is a big BUT : this one is not yellow.
The screen is so white i can't even find yellow tint, and comparing to my friend's iphone 4, (his screen is really violet / blue) there is a difference, but mine doesn't draw yellow tint, but a clear white tint.
And compared to the iphone 4s of my other friend (the DNP one i posted spec page 19) which was clearly white compared to my other's 4s, this new iphone i just had is a bite whiter than his.
So it's possible to have a white tint and a C39 serial. I just think display are badly/randomly calibrated at factory, due to lack of orders from apple, or quality control.
Seriously, when the genius did the last exchange (for me, because it's really a waste of time to stay 2 hours at apple store each week for that), and when i saw the C39 serial on the black box, i was frightened, but finally it's ok.
I have the battery problem, as ever, but it seems to be a software problem, so it will be fixed.
Finally, i'm happy ^^
Note : All exposure models at apple store were C38 serials models (i checked 4 or 5 models ), and they were perfectly white, and they had only C39 refurbs models (told to me by the genius, when i asked to have a C38 one) And this time the genius was really friendly with me compared to the other time. (Apple store Opera, in France)