Thanks to HTC Product Manager Leigh Momii who tweeted that the AT&T version of the HTC One (M8) will be on the receiving end of the Android 4.4.3 KitKat update some time later today. So far, AT&T and HTC have not put up any kind of official changelog for this particular update, but it ought not to venture too far away from the same update that hit the other HTC One (M8) models such as the relevant (and necessary) security fixes, better Wi-Fi and Bluetooth connectivity, enhancements to the camera, as well as optimizations made to the different features found on HTC Sense 6.
To look for the update on the AT&T HTC One (M8), go to Settings > AT&T Software Update > Check for updates, and be patient.