Quick heads-up!! Though AT&T has mentioned Lumia 520 availability from 26th July in their press release, but it seems device can now be booked online and can be purchased from store as well. The pricing is amazing at $99 and no need to remind you that HSN sold more than 22000 Lumia 520 at this price in one day.
According to the tip from Joeri,
lumia 520 available on AT&T ( if you click “check in store availability” & enter 71115) you see that that lumia 520 is in stock.
Thanks Joeri for the Tip.