I was told by AT&T store employees that you can't port your number from another provider is you start a subscription at the Apple store.
A couple months after the 3GS came out, my family was shopping to bring two lines from Verizon over to AT&T and get two iPhone 3GS 32GB models. The Apple store employees said, 'That's great. You'll still pay full price for everything no matter what you buy'. The AT&T store was very willing to haggle. "you're buying two iphones, alright we'll waive the $36/line activation fees. You're buying Applecare and MobileMe for each iPhone, we'll give you a car charger, pack of screen protectors, and your choice of any case in the store for each iPhone for free. You're buying all that stuff, we'll give you a third new phone for another user in your family that has a two-year contract price of $199, for free, with free activation".
So yeah, at least when supply strengthens, you'll probably be able to haggle free activation and maybe some accessories depending on what you buy and what your phone plan is. At the Apple store you will not. However, I will say that my choice of the "best iPhone case in the AT&T store" turned out to be useless. They don't carry any higher end cases. The Apple store is much better at stocking higher quality, albeit more expensive, accessories. But they won't give you any of them for free.