Here's the thing - the 2 year agreement is a contract that you'll remain an at&t customer for 2 years. They could care less which phone you use. Normally the only reason you have to wait 2 years to get a new phone is because when your contract is up, you are eligible for the $200 subsidy toward a new phone. If you want to forgo that subsidy (or pick up a secondhand at&t or unlocked phone on ebay), you can change phones every week if you want to.
Is there any reason to think that the at&t store won't sell the iPhone without requiring on-the-spot activation? My guess is that they'll be happy to help activate it if you so choose, otherwise you'll be able to buy the phone, take it home and activate it there, just the same as buying it anywhere else. I'm pretty sure they don't want the nightmare of having to go through and activate the phones for the masses of people that will be lined up outside the stores, especially if there's an easy apple-ish way of activating it yourself, which there almost certainly is.
Is there any reason to think that the at&t store won't sell the iPhone without requiring on-the-spot activation? My guess is that they'll be happy to help activate it if you so choose, otherwise you'll be able to buy the phone, take it home and activate it there, just the same as buying it anywhere else. I'm pretty sure they don't want the nightmare of having to go through and activate the phones for the masses of people that will be lined up outside the stores, especially if there's an easy apple-ish way of activating it yourself, which there almost certainly is.