The stores have a quantity being shipped that corresponds to the size of the store. Standard Apple stores: 250-300, flagships: 600-750;
AT&T stores should be similarly stocked, but scaled down to the size of their stores. Because Apple can handle a greater volume of customers (at once) - more iPhones are going to Apple retail stores for the first two weeks.
Words of reason.
Also - what's with people advising others to go to a store in, like, Beaver Bottom, Georgia because there won't be as many people waiting there? Then there won't be as many iPhones there either. Apple and AT&T know better than to send products to stores where they won't get sold. You'd be surprised how well stores can forecast sales.
Any limit in availability of iPhones will stem solely from production difficulties, not from some artificial restricting.