Like every other U.S. carrier (except perhaps Verizon, which hasn't really given a launch date yet, other than to say preorders will ship around July 9 or 10), AT&T's found itself in a pickle with the Galaxy S III. There just aren't enough to go around. We've finally gotten an official statement from the carrier, which reads thusly:
Manufacturer supply constraints of the 4G LTE Samsung Galaxy S III have caused a delay to expected pre-order shipments. Customers who were previously quoted a delivery date of June 21st should now expect their device to arrive no later than Monday, June 25th. We currently estimate future pre-orders to arrive within 10 business days based on available supply. Impacted customers will receive both an SMS and email notifying them of this change.
So, if you've preordered and were given a June 21 date, look for it by Monday. (And it wouldn't surprise us in the slightest to see phones hitting doorsteps before then, as some folks in our forums are expecting.) If you preorder today, chances are it'll take longer, but AT&T's now saying online that it should ship within 10 business days.
You'll note that there's no mention of in-store availability. We're pressing on that, but it's easy, folks. Call the store before you go. Use this link to find your local contact info.