It looks like the Korean-language version of the official Diablo III website may have jumped the gun a bit on confirming another character class for the upcoming Blizzard title.
The page dedicated to the game's character classes showed the Barbarian, Witchdoctor, Wizard and Monk, though the final, still unconfirmed class, didn't show any additional information when it was clicked on.
It shouldn't come as a surprise that the Monk appears to be coming to the latest iteration of Diablo. The Monk was a playable class in the Hellfire expansion for the original title along with the Barbarian, Bard, Rogue, Sorcerer and Warrior.
I suspect we'll be getting some sort of official confirmation on this in the coming hour or so with Blizzcon just moments from going live.