Go fire it up and give it a try.

You now have the chance to try Blizzard's new e-sports experiment, the Arena Tournament realm, on the public test server.

The new Arena Tournament realm for World of Warcraft is available for testing on the Public Test Realm. Join your fellow players to be first to experience this exciting new form of PvP. Please also provide us any feedback at the Arena Tournament Forums. More information can be found at the Arena Tournament information page, item list, and FAQ.

Stop by the page they mention for more info.