I got the email with the trade in code but if you go to an AT&T corporate store they will give you the card right there. They gave me the $100 credit for the iPhone 4S but then added $100 more because I ordered my iPhone 6 Plus prior to 9/30.
I traded in my old unlocked AT&T iPhone 4 for $200 at a local corporate store in September, right after the original promotion started but before I'd ordered my new iPhone 6. After receiving my new phone a couple of weeks ago, I got the email with the trade-in promotion codes. Just got back from my local AT&T corporate store after trading in a second iPhone 4 (this one a Verizon unit) for another $200 promotion card.
I had previously been told that AT&T will allow 4 phone trade ins per year, but wasn't sure that they'd do more than one under this promotion...