Blizzard has announced in its Battle.net Blog that the 5.0.4 update which lands on the 25th August will unlock every single World of Warcraft race to all users regardless of which expansions they own - including the Pandaren race that will be introduced on the 25th September in the Mists of Pandera expansion.
This also includes the Draenei and Blood Elves from Burning Crusade and the Goblins and Worgen from Cataclysm.
Of course, it seems what the patch won’t unlock is the Monk class being introduced in Mists of Pandera.