Yes, the very same SEC filing mentions that Blizzard is currently hard at work on Cataclysm, StarCraft II and "sequels to the Diablo franchise." It's very possible, and even likely, that the filing refers to Diablo III and its inevitable expansions (which had better include the Necromancer). It's also possible that the phrasing is meant as a forward-looking statement, stating that Blizzard continues to work on the Diablo franchise. It's also worth noting that the same SEC filing refers to "development of a sequel" to the game, so reference to "sequels" could simply be a mistake. Or maybe, just maybe, Blizzard is pumping out more Diablo than we thought.
When reached for comment, a Blizzard representative told Joystiq, "We don't have any new announcements to make." He added, "For the Diablo franchise, our full focus is on Diablo III." So that means it's coming out really soon, right? Right?