From wowinsider.com, Tigole (Blue Post):

We'd like to thank players for participating so zealously (and valiantly!) during the Great Zombie Plague of '08. Our intention with this event was to create something memorable and to literally change the face of Azeroth for a few days. This event is just one of many leading up to Wrath of the Lich King. As you know, the Scourge Invasion is now live as well and there is some other cool stuff coming up before all is said and done. Also, Hallow's End continues on!

While we recognize that the event could prove to be disruptive at times, we hope you made the most out of it while it lasted. Part of playing in a "living and breathing world" means that things are going to change from time to time. We meant no harm but only wanted to create a lasting impression on players as they head to Northrend to defeat the Lich King. The Lich King's goals (as you will soon learn) are to turn the Horde and Alliance not only on one another, but on their own kind as well. I think the plague truly demonstrated this firsthand.

So it looks like a cure has finally been found… for now.

Thank you for participating in our most memorable event to date… for better or for…. brrrrrrraaaaaains…

http://www.wowinsider.com/2008/10/27/the-undead-plague-ends-today/

Sounds like there is more in store...