An unlocked iPhone 4S were 'officially' came out in November last year. We want to remember you once more that in accordance with 9to5Mac, if you purchased an off-contract
unlocked iPhone 4S from AT&T, your device is definitely unlocked.
"Off Contract" iPhones 4S are sold at full market price and do not lock you into a contract with AT&T. The costs of these models are $649, $749 and $849 for the 16GB, 32GB, and 64GB iPhone 4S models.
The discovering was mentioned by 9to5Mac and had already been talked about in the MacRumors forums where a variety of users realized that iTunes informed them of the "Congratulations your iPhone has been unlocked" message from iTunes:
“
I really have no idea. Part of me thinks it was some kind of glitch. The only thing I can think of is that a paid full price and didn't extend my contract. I ordered through Apple too. I wish there was a way I could confirm it, but I don't have any T Mobile or foreign sims laying around lol - BFW122083
The same thing appeared on mine when I did a restore... I am wondering if it came like this from the factory because I paid the full $849.99 for a 64 GB because my AT&T contract was not eligible for an upgrade...”- mjcxp
The most popular thread was that they were purchased as off contract
iPhone 4S released recently. Developer @stroughtonsmith has since confirmed this by getting one himself.
At the same time, it appears Sprint iPhone 4S are also come GSM unlocked based on @gruber who was able to correctly use a Canadian SIM card in the Sprint iPhone 4S. Even so, Sprint has guaranteed to lock them later on.