Posted regarding this on our forums:

Sorry about the TBA message! We're working on implementing the Blizzard card back and Warlords card back, but some switches were flipped a little too early.

Getting the TBA message isn't a bad thing, and it won't prevent you from getting those card backs. It's just a visual error. The only thing that you might experience is once those card backs go live and are available to acquire, you will not get that notification again. The cards backs will be waiting for you where all the other card backs live - in the 'Options' menu.