I also received $100 back from AT&T for my 1520. I had an early online pre-order from before the "$100 off any smartphone" sale, which I decided to keep even after the discounts were announced, rather than cancelling and having to re-enter the shipping out line. To my pleasant surprise, I received the following email a few days after getting the device, without me even asking for it:
AT&T
Dear...,
Thank you for pre-ordering the Nokia Lumia 1520 from AT&T. We are excited to announce that after your Lumia 1520 ships, we will apply a credit on your order for $100 due to a promotional price decrease after you ordered. The credit will be applied to the payment method used for this order.
We appreciate your business.
Sincerely,
AT&T