Upper Deck's Darkmoon Faire event before, it's a pretty good time. It's obviously a lot more fun if you're a fan of the WoW TCG (as they have open-to-close tournaments and events going on), but even online game players will find something to do -- UDE sets up an event realm provided by Blizzard, and they sometimes even have leveling and instance contests as well.
Upper Deck is holding a Darkmoon Faire event in Seattle next weekend, June 13-15, and they sent along a note letting us know that the guys from Penny Arcade are going to be there in attendance. Both Gabe and Tycho, you'll know if you're an avid reader of the comic, are fans of the card game, and while Upper Deck isn't clear about what they'll be doing there (all they tell us is that the guys will "partake" of the gaming), it's not hard to expect that at least Tycho will be eventually running a Molten Core raid game.
Should be fun -- if you're around Seattle and get to stop by the Seattle Center next weekend (it's free to visit the event, and various tournaments have different costs associated with them), definitely take pictures and let us know how it went.