It appears the pre-launch World Event for Wrath of the Lich King is now active on all US and EU servers! (Unknown if active on Korean and Chinese servers)
What is known right now is that it seems to be phase 1, which is a zombie invasion. Players can become infected by Conspicuous Crates (or other zombies), and the Argent Dawn has dispatched agents across Azeroth and Outland to help combat the threat. You can find them stationed at Argent Dawn banners.
If the disease isn't cured by either a player or an Argent Healer, you're transformed into a zombie and can attack other players -- infecting them in the process. While transformed, you gain access to special abilities only available while zombiefied, and you're also friendly to other zombies.
We're working on getting more screenshots now, and will continue to provide updates as we learn more about the event and the phases that unfold.