Arthas just backed the moving truck in and he is here to stay.
Apparently Arthas can't just move in down the street and keep to himself in Northrend. No, he needs to announce his presence with authority (a la Ebby Calvin LaLouche aka Nuke in Bull Durham).
Stories are popping up all over about a zombie plague infesting World of Warcraft in a way similar to when Naxx showed up the first time. You can catch the plague in a number of ways (like opening some mysterious boxes in Booty Bay) and you have 10 minutes to cleanse the disease, or you turn into a zombie that is hostile to all factions except other zombies. If you do turn into a zombie, you get a zombie-type spell bar yet none of your character's standard actions.
Why do I see some concerted zombie zerging of major cities soon?
Good luck out there, it could get ugly.