UPDATE #2: Screenshots added as well as initial info on playing as a zombie. UPDATE: We have information on how to become infected.  The source is from Booty Bay -- click for more details.

 

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.

How to become infected Travel to Booty Bay and right-click a Conspicuous Crate.  This will infect you and open up a window with some flavor text.  After 10 minutes you will transform into a Plague Zombie -- which uses the new vehicle system -- and unlocks special zombie skills.  Dying to an NPC or player while infected, or using immunity abilities such as Divine Shield will instantly transform you.
Caution, NPCs can become infected too, and they will transform into a ghoul of the same level.  This is especially dangerous for areas with guards higher level than the content, such as Booty Bay, the cities, etc.
While infected, even going near an Argent Healer will cause them to cure you.  If you want to play around as a zombie, use caution and don't approach them.
From all accounts thus far, you become a zombie of your level, so low level players will have lower health and overall stats as a zombie than level 70 players.
Playing as a zombie While a zombie you unlock special abilities only available in this form.
Your zombie transformation has no set duration, but you are under the effect of a constant DoT that will kill you if you do not feed.
You must attack other players or NPCs to feed; doing this will proc "Bite" which heals you.
Your zombie stats are not dependant on your items at all, so you can unequip them to avoid durability loss.
Dying to other players does not cause you to lose durability, but dying to NPCs will cause durability loss.
It appears that you can only infect humanoids, and possibly only humanoids who are KoS to you while a zombie.
You are only able to attack players of your faction.  (Maybe a bug?) This appears to only be an issue in Shattrath City.  I was able to attack Horde players in other areas.  Shattrath is being really weird about who you can attack in zombie form (and what zombies you can attack).

World of Warcraft, Wrath of the Lich King, Wrath of the Lich King Launch Event
Article Source: www.curse.com.