According to a tweet by community manager Zarhym, the dungeons opening up this afternoon are Mogu'shan Palace and the Shado-Pan Monastery. Keep an eye on WoW Insider -- we'll supply you with all of the details as they become available.
It's open warfare between Alliance and Horde in Mists of Pandaria, World of Warcraft's next expansion. Jump into five new levels with new talents and class mechanics, try the new monk class, and create a pandaren character to ally with either Horde or Alliance. Look for expansion basics in our Mists FAQ, or dig into our spring press event coverage for more details!