As my nexus 4 has been shipped and will be arriving soon, I went to order a sim from straight talk and here's no option for buying an AT&T sim anymore. I do some searching around and it seems they stopped selling them online about a week ago. There have been conflicting reports on whether they will be selling them again or if they're going TMobile only, but on twitter they have said they will be selling them again once they become available. TMobile is complete crap where I live (upstate NY), with coverage being mostly 2G, 3G in some towns, and a lot of no coverage areas.
Apparently they are still honoring AT&T sims if you get them from Walmart, so I'll be checking out the local stores. Thankfully they're currently listed as in stock online but that may not last, as they were listed as out of stock recently.