With the upcoming Patch 5.0.4 for World of Warcraft, every race will be playable for free, no matter which version or expansion pack players own. The patch includes the Pandaren race, regardless of whether you purchase the Mists of Pandaria expansion or not.
"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," a Blizzard spokesman said on their blog.
The patch will release on August 28th, but since Mists of Pandaria does not release until September 25th, reason would have it that the Pandaren will not be available until the expansion pack launches next month.