When Att likes an OS build enough to support it and release it. Unless you personally know the Att software testers (I'm assuming you don't since you asked the question) then we have no idea when that will be...
I'm about 98% sure they won't release OS6 for the 9300 before they release it for the 9700. It's hard to say when they'll do that either, given the 9780 coming out. It took them over a year to upgrade the 8900 from 4.xxx to 5.xxx
In saying, that, though, hopefully we will see it soon. Though I'm guessing it won't be any sooner than a month or two away.
