I'm going to try not to speculate too hard on what kind of in-game event Blizzard might do for Hearthstone, but it would be really cool to see, say, a bunch of NPCs hanging around in various Azerothian taverns playing card games and talking trash to each other. Hint, hint. I'm just saying! Of course, we all have to remember to take this with the requisite datamining disclaimer: just because it's been found in the PTR files doesn't mean it will make it to live. I really hope it does, though, because I'm always happy to grab more mounts!
Edit: Thanks to all who have pointed out that this may very well be the participation reward for the Hearthstone beta. This is what I get for not rereading the Hearthstone FAQ before submitting a post!