It could be a reward for making a purchase once it's live, but I doubt it. I think it'll be earned in the normal course of gameplay in hearthstone (possibly in arenas).
It was originally supposed to be reward for those participating in HS beta, but that idea was scrapped. Noone knows what it'll be for now.
Unless I missed something I think everyone just presumed that it was for participating in the Hearthstone beta.
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