Incorrect, unlocked phones are not available on the 21st. Contract free, full price are but unlocked aren't slated until later.
Like when the 4S last year?
Apple won't officially call it unlocked, but if you go into the store and buy the AT&T version for full price ($649 for the 16GB version), it
will be unlocked. That's what I did last year with the 4S (a few days after the launch but still before the official unlocked one went on sale).
You can also go to an AT&T store and buy it at full price, but then you'll need to ask AT&T to unlock it since the phones they get are locked to their network by default. Their online form claims they will unlock for non-customers provided you give them proof you bought it from an authorized retailer and are not trying to defraud AT&T (basically it's there to scare away would-be scalpers).
The thing to keep in mind is that at Apple stores, the phones are "unlocked" by default until Apple registers it with a network on their database, at which point it is locked to the network. When you buy a phone, normally Apple accesses your carrier's server to verify the subsidized price to charge you. If you tell them to sell you the AT&T version at full price, they don't need to do that, and thus won't enter it into their database as locked.
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Incorrect, unlocked phones are not available on the 21st. Contract free, full price are but unlocked aren't slated until later.
Technically yes, but if you purchase a full-priced iPhone from an authorized seller (and Apple stores most certainly are authorized), you can request that AT&T unlock it.
https://www.att.com/deviceunlock/client/en_US/?