After T-Mobile and Verizon, the AT&T variants of the Samsung Galaxy S8 and S8+ are also being updated. Weighing in at around 300MB, the update is arriving as version G950USQU1AQD9 for the former and G955USQU1AQD9 for the latter.
Sadly, there's currently no information on exactly what the update brings along. The official change-log for the T-Mobile update was also not available, although Verizon had detailed the changes included in its update.
Still not available on my AT&T S8 either by OTA or by Smart Switch. Still on QC9.
Simply amazing - AT&T published an update in less than 6 months.
It would obviously be a software update not a hardware update. Phone manufacturers tend to release an update for a brand new phone shortly after release to fix any bugs that may present themselves upon release of the device.