More Card Backs Coming Soon!
Zeriyah posted some information on the forums regarding new card backs! They have released details on how three of these will be obtained, but the others are "upcoming rewards". Check out below for the cards and the blue post.
Reaching Legend Rank
Ranked Participation Reward
Promotional Events
Upcoming Reward
Upcoming Reward
Quote from ZeriyahComing soon to Hearthstone – a new way to show off your proudest accomplishments in-game with alternate card backs! Decorate the backs of your hard-earned cards, each with unique art and a different way to acquire them. You and your opponent can see your card backs in-game as you duel! Collect them all!
Be sure to keep an eye out for new card backs to add to your collection!
See all of the different card backs here. Click on each card to see how you can get it!
Alternate Imgur album link for non-Facebook users.
There is no rewards during the test seasons/open beta, so these are for the first real season.
As for the other two, I figure at least one will be somewhat related to the first adventure that gets released.
So far it looks like any rewards will not be applied to past achievements.
Question: "Does anyone know if we get the legendary card backs from the legendary status from this season? Would be cool." Reply from Blizz employee: "Not from this season - we'll announce when they go live." - CM Zeriyah