My guess is that you can get a full-priced AT&T version and get it unlocked. AT&T is doing that now. I'd give them a call.
Anyway, last year the unlocked models went on sale about a month after the carrier-locked models did. I see they still offer the 4S unlocked, so my guess is that they will do the same, but haven't yet announced a date as they want to get the main rollout done.