Luckily, this tradition continues in Mists, and it's even easier to get started. Once you're 90, head to Kun-lai Summit and the Temple of the White Tiger. Gurgthock will be standing next to a gong just outside the temple proper, and he'll give you a quest to enter the Arena of Annihilation, with the reward being the choice of a couple weapons tailor made for your class.
Once you get the quest, what you'll want to do is queue for the Arena of Annihilation scenario. Open the Dungeon Finder Interface (The hotkey should be "I" by default) and choose the Scenarios option. Choose Specific Scenarios from the drop down menu, check the box next to Arena of Annihilation, make sure no other boxes are checked, and hit Find Group. Within moments, you'll be in the Scenario and well on your way to your weapon. Once you defeat the final boss, Wodin the Troll-Servant will be waiting right there in the scenario to reward you.
The weapon will be a nice, solid rare weapon of ilvl 450. It's not as good as a heroic dungeon drop, but if you just dinged 90, it's bound to be better than the quest green you're likely using, and it'll help boost your item level so you can run heroics that much faster.
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!