MMO-Champion reports that "Mists of Pandaria" is "extremely likely" the next WoW expansion. While some folks are unconvinced, I'm inclined to trust the MMO-Champion track record.
Hindsight is always easier than foresight - my best guess would be nowhere near the mark - but this makes a lot of sense in hindsight. Regardless of the success or failure of Cataclysm's world revamp, which peppered with a few new zones in previously blank areas of map, the more traditional model with a new continent makes sense as a follow-up. If Pandaria does turn out to be located on a previously unknown South Seas island, it is a logical battleground for an invasion by Azshara and the Naga. There are relatively few other combinations of location and antagonist that players have heard of (especially through the Vashj'ir storyline in Cataclysm) and that are not similar to past foes (The Burning Legion again, or yet another Dragon Aspect gone evil).
There are a few obstacles Blizzard has to deal with, if this is the expansion. Panda haters are going to hate, and Panda lovers aren't going to be happy unless the race is playable for both factions, as Rohan suggests, which would be a first. That said, I see no niche for a Brewmaster class, even if Blizzard does want to take the PR hit for marketing drunken cartoon pandas in a game that kids play, which might mean no new class. WoW expansions have always included either new races or a new class, and Blizzard had previously suggested that two new races every expansion would be tough due to art requirements. One new, neutral race mitigates that concern by halving the art requirements, even if two panda factions require Alliance and Horde colors.
There's also the issue of China, which was rumored to have prompted the removal of the Pandaren as the Alliance race for the 2007 Burning Crusade expansion. Six months ago, I would have guessed that Blizzard no longer cared about China due to the likelihood that the government would refuse to approve the next expansion no matter what its contents. However, after a rocky period that saw WoW China shut down outright and the Wrath expansion delayed by nearly two years, Blizzard seems to have finally mended relations; Cataclysm reportedly hit China a mere seven months late. I don't have a good answer for this question, other than that Blizzard apparently thinks they can do something involving Pandaren - perhaps the Chinese version of the expansion will have all the Pandas find-replaced with Worgen?
In the end, my guess is that Pandas, much like EQ2 Beastlords, will arrive because people want to see them. Yes, some people will argue that this is WoW jumping the shark, but others have been asking for the Pandaren at every Blizzcon since the first. By the time you're talking about a seven year old MMO, your target audience really should be your current playerbase. Very few players who have stuck with the game through Cataclysm are going to walk off because they think Pandaren are a joke, while players who have already made the decision to leave are free to mock the move as validating their departure. Sounds like as good a choice as Blizzard could make under the circumstances.