I was under the impression that Denim wasn't available on older at&t Lumias in the U.S. yet, until the following happened: My main phone is a Lumia 1520, but I have a 920 and a 520 as well. Anyway, I pulled out my 920 and reset it. When I did (after all the apps updated), it showed Lumia Denim. Once I installed Preview for Developers and updated the phone, it disappeared and returned to Lumia Cyan. I then reset the 520 and the exact same thing happened. The 520 now shows it's running Denim. Can anyone shed some light on this?