"With the release of patch 5.0.4, we’re making all the races of Azeroth playable by anyone, no matter which version or expansion of World of Warcraft they own. This includes the enigmatic pandaren, who will become available for play when Mists of Pandaria is released on Tuesday, September 25. We want the entire World of Warcraft community to be able to embrace their inner goblin, blood elf, worgen, draenei, or pandaren from the moment they step foot into the world of Azeroth.
Now, the only hard part will be choosing a name. Who will you be next?"
UPDATE: Zarhym just put up a clarification statement:
"I've seen some confusion about when pandaren character creation will be accessible. Just to be clear on the announcement, we're making it possible for people to create a character of any race, regardless of what expansions you've purchased. So if you only have the original game (which now has The Burning Crusade included), when patch 5.0.4 hits you'll be able to create goblin and worgen characters as well. And, once Mists of Pandaria is released, you'll be able to create pandaren characters, regardless of whether or not you own Mists of Pandaria."