Pretty sure you can't buy a couple a week. You need to buy them with an existing account (or start a new one). Say you buy 5 phones, that account will be marked saying you bought 5 phones. When you buy them, you're starting a new 2 year contract (even at the Apple store, the POS ties into the AT&T system). They're not going to let you buy more phones the follow week because it shows on your account that you just bought 5 phones. And if you cancel the account, they'll make you return the phones because it's less than 30 days. After 30 days you can cancel and keep the phones, but have to pay the ETF. Unless there's some really crazy hidden backdoor way around it, your best case is getting new phones every 30 days. And even then you have to keep breaking a contract and getting a new one, and eventually they'll probably just deny you, not to mention the hit your credit score is going to take from constantly having them pull your credit (ever new contract they'll do it) and canceling contracts.
I don't see how you're possibly going to make any money off this when you factor in the cost plus fees and stuff. As a 1-time deal you'll probably be ok, but if you repeatedly do it, I can't see it being worth the hassle?