Hm, does this work only if ordered through AT&T's web site, or can you order through Apple and still get the promo? I'm guessing not.
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Also, the fact that it's only good for purchases at AT&T is kind of a blower. I'd want to buy the phone itself on my own card for my credit card benefits, which means the trade in card would only really be good for paying your bill. And as far as I know, there's no way to pay a single bill using multiple cards. So unless $200/$300 is a perfect multiple of your monthly bill, you'll have some unusable balance left on the card after paying a few bills.