From TUAW:
I'm going to speculate this ...
AT&T Stores will do everything in-store ... Apple Stores will give customers the option whether to activate in-store or at home. From that AT&T memo, the current iTunes activation process "will no longer be required" but it still exists "in the event that a customer's device is not tethered in the Apple Store."
Maybe, just maybe, existing iPhone customers with a post-paid plan will be given the option of buying and activating at home. That could mean really short lines for us at the Apple Store on day one.