I just went online to check out the 4S, and noticed that the models all feature iOS 7 screenshots. I would imagine it is likely if I buy a 4S it will come with iOS 7?
I would say the 8 GB model would definitely have iOS 7, as it came out at the same time. If you get the older 16 GB model (or higher) then it may come with iOS 6.
I got a replacement 4S (64 GB) a couple of weeks ago and it had 6.x. So did the second replacement (the first was defective), but they upgraded me to 7.0.2 before I left the store.
I would add that both phones were manufactured in September.