That the Pandaren Monk would sell out comes as a surprise because, well, it's a virtual item. It's like Agent Smith. It never runs out. Unless, of course, Blizzard designed it that way, in which case there must have been a massive failure of communication somewhere down the line because it doesn't say anything about the Pandaren Monk being a limited item anywhere. It's also notable that Blizzard had committed to donating 50% of all proceeds from Pandaren Monk sales to the Make-a-Wish Foundation all the way through December 31. If the Pandaren Monk were to disappear, it should at least be expected to last until the end of 2009, right?
We think this is an inventory error, where some number had to be inserted into the Blizzard Store's inventory management system, like some number "over 9000". Likely this will be easily corrected, and when it is, we'll update this post accordingly.
UPDATE 11:30am ET: Blizzard is looking into the matter, which should assuage most people's fears that the item is indeed sold out.
UPDATE 2:00pm ET: The Pandaren Monk is now back in stock and available for purchase! It was just a glitch in the Matrix, people. Nothing to see here.
Thanks to Papa Shok for the tip.