I only found out about this yesterday, but I definitely intend on sizing up an opportunity to have a long session in The World's Largest Dungeon to celebrate the occasion.
Me and a couple of the guys from my group are going to one of the official events over in Johnson City, TN since neither of the local gaming shops are participating.
I think we're playing in the Eberron adventure. Not my first choice, but at least I can say I've tried it now when I say I don't like it.
I'm going to be running the events at the store I work at. That reminds me, I need to get to work on the pregens I'm going to do for Forgotten Forge. Hmm. And scale the thing up to six players.
I'll probably just have a quiet (well probably not quiet) game with some friends/family. We'll probably order pizza or chinese and kick into Ebberon big time.
Just my little way of observing World Game Day, I suppose.