Much like the Operation: Shieldwall and Dominance Offensive quests from 5.1, level 90s will automatically get the "Thunder Calls" quest upon entering the Vale of Eternal Blossoms. Alternately, you can go to the Shado Pan Garrison at Townlong Steppes to pick up the intro quest. After that, you'll be tasked with making it to the Isle of Thunder on your own -- and it won't be easy! After your first trip, the Shado Pan will provide a portal to the appropriate faction destination for your further questing convenience.
I'm more excited for patch 5.2 than I have been for a WoW patch in a good long while; probably since Icecrown, to be honest! Lei Shen, it's time to face the might of the heroes of Azeroth!