A post on the Hearthstone: Heroes of Warcraft Facebook page has unveiled several alternate artwork options for the backs of your cards. They are colorful and fun, and will be visible to both you and your opponent while you play a match. You can earn the card backs by attaining certain ranks in ranked play. Thus, your card backs can serve as reminder to your opponents just who they're up against.
Hearthstone CM Zeriyah has also let it be known that the new card backs will be animated, though we don't yet have a preview of that. Currently, my favorite of the lot is the rainbow (who doesn't love rainbows? Besides imps, anyway), but I'll withhold my final judgement until we get to see just what the animations look like. I'm sure more unique, animated card backs will be on their way in the future, as well, and they'll provide a nice goal for collectors and competitive players to work toward.
Gallery: Hearthstone Card Backs