Showing up quickly after the international Xperia Ion started to receive an Ice Cream Sandwich update, the AT&T model of the same phone has started to show up in SuS -- Sony's PC companion software -- for some users over in the XDA forums. The AT&T model, LT28AT, is being pushed up to software build 6.1.c.1.105 which brings all of the features and visual cues associated with ICS. We won't know for sure what other fixes or changes are included in this update until AT&T spills the beans officially.
We'll see if the update starts to roll out to more users in the next couple days, and hopefully it'll start to come out over OTA rather than relying on Sony's PC software. Check your own device and let us know in the comments if you're seeing the update.
Source: XDA Forums; Thanks Terry!