The deal is available whether you are activating a new AT&T line or upgrading a current account. Despite the special pricing, the 14 day return policy remains in effect. And you don't have to decide to buy the discounted phone right away. If you buy a qualifying Nokia or HTC model and decide anytime up to February 21st to add a second phone, you will get the $100 discount.
You need a pair of Windows Phone 8 handsets? You could save $100 if you make the purchase from AT&T. And speaking of free, Nokia and AT&T have extended their offer for the free Qi-based wireless charging plate for buyers of the Nokia Lumia 920. The offer was set to expire on January 10th but now has been extended to February 21st.
source: AT&T via WPCentral