It looks like the HTC One (M8) isn't the only handset sold by AT&T that's getting a new Android version today. The latest update to Android 5.0 Lollipop is now rolling out to Samsung Galaxy S4 devices bought from AT&T. The Galaxy S4 has been on the receiving end of this tasty treat for a few weeks now, but in different regions or at different carriers.
If you own a Galaxy S4 purchased from AT&T, you should be notified about the update within the next few days. If you're impatient, you can manually check by going to Settings > More > About device > Software update.
The download comes in at 947MB, and you must grab it via Wi-Fi. After applying the update, your phone will run Android 5.0.1, build number LRX22C.1337UCUGOC3.
Aside from everything that Lollipop brings with it, you obviously also get Samsung's refreshed TouchWiz UI - with punchier colors and whatnot. In this software release, AT&T has removed the Code Scanner, Browser Bar, and Softcard apps - so you'll have less bloat.
Thanks for the tip, Kurt Reardon!