Turns out that the Diablo III game isn't the only big news Blizzard will be announcing this year. They've got another MMO they've been working on. Rob Pardo spoke briefly on the subject saying “So we have another development team. So yeah, there's still an unknown project in development from us.”
Beyond that there are no details on what this project could be. Perhaps it could be that they are developing an MMO that actually could be ad-supported like they had originally intended WoW to be.
They could also be taking one of their games that is not an MMO currently and changing it into one. There is no real way to tell, but hopefully we'll find out at this year's BlizzCon in October.