Software update done for Sprint and Verizon, and started for Tmobile. For AT&T...? Next week.
What did I tell you. AT&T would be last, and just about two weeks before it finally comes (said this last Friday). I expect Tuesday to be the day it's available. Not sure about a mass push that early, but it might be slow. This may be much like what happened on Sprint (available to pull for most, then full push shortly after).
You never know for sure, I mean, I don't work for AT&T. Monday morning could be where everyone gets notified on their HTC One handsets. Backup and prep your phone's for the new update. Three main points: Loss of Flash Player; Remove Option for BlinkFeed; Lock Screen Widgets (Built-in).