Straight Talk web site. After initailly saying the SIMs were simply out of stock, Straight Talk representatives have confirmed the AT&T SIMs have been discontinued.
No explanation for the discontinuation was given, leading to speculation that America Movil, the parent company of NET10, Straight Talk and Tracfone) and AT&T are involved in some sort of contract dispute. If that's the case, AT&T may have blocked the sale of the SIMs or America Movil could have removed them from sale as a negotiating ploy.
Straight Talk seems to be directing customers away from AT&T based products as much as possible. My market (San Francisco, 94103) has been reclassified from GSM4SPRCO (AT&T, Sprint, Verizon) to GSM5SPRCO (T-Mobile, Sprint, Verizon) meaning that no AT&T based phones are available to anyone who enters a San Francisco zip code on the Straight Talk site.
Straight Talk and NET10's AT&T based phones including Android and Symbian smartphones are still available in zip codes like 65046 where AT&T is the only carrier.
If you must have a Straight Talk AT&T SIM they can still be found on eBay, but expect to pay $50 for more for one.
An alternative is Straight Talk's sister brand NET10. AT&T based Net10 SIMs are readily available for $5 or less from Amazon and from well regarded eBay seller ShopCellDeals. NET10 itself will also sell you an AT&T SIM in your use an AT&T only zipcode like 65046.
With Straight Talk ending and Net10 cutting back their AT&T based bring your own phone program, what will happen to existing customers? According to Straight Talk and NET10 representaives current customers can continue using their SIM indefinitely and unactivated AT&T SIMs can still be activated. However NET10 has imposed a 1.5GB/month cap on AT&T SIM users and Straight Talk is reportedly throttling AT&T users who use more than 50MB in a day.
For alternatives to Straight Talk and NET10 for AT&T based service see: AT&T and AT&T MVNOs Compared.
Updated 5/22/2013 with current information on SIM availability and NET10 and Straight Talk data caps and throttling.