I was shopping this afternoon and stopped into the Microsoft store.
I talked to a couple store reps for some time about the release date. They told me they don't have an official date, or can't really talk about that. However, 2 of the reps (both with AT&T one a 1520 & the other with a 925) showed me their phones and said the update had started rolling out, because this morning their phones were updated to 8.1
.
I don't think they were running the developer preview, but both phones were defiantly running 8.1. Could be because they are employees, but I really don't know, has anyone else on AT&T been updated. I just know it's not here for Verizon yet.