Currently in-contract with AT&T, not eligible, and I'm buying a white iPhone... what's required that I bring to the Apple Store?
Well, I bought a black one so this may be completely different but I brought my drivers license and credit card. I showed him my license and paid with my credit card. Chances are its the exact same stuff you brought when you signed up for AT&T the first time. Or, like, whenever you drive to buy anything.
I am going to go out on a limb here and say you should probably take what is on the list on AT&Ts website and in all the stories here on macrumors...