And surely Blizzard learned a lot from what happened a year ago -- Linedan has a nice wrapup of the good and the bad that Blizzard may have taken away from the zombie apocalypse. The idea of turning players on players was great, but that unfortunately led to more griefing than most players would have liked (and the fact that, by the end of it, you couldn't avoid the zombies at all, probably didn't help). And for all of the disruption, there was no real reward (the eventual rewards came with the Scourge invasion, and then it was simply just farming tokens), and no real payoff (the final world event seemed half-cooked, and it was only implied that Arthas was the one who'd shipped the crates).
So hopefully Blizzard will take all of these lessons and all of this experience, and really impress us with the upcoming Cataclysm world event. I have to say, it'll be tough to beat anything with zombies in it, especially an event that spreads like a plague and gives us a whole new set of zombie abilities to run around and eat brains with. But we can't wait to see what they'll try.