Gamona interviewed Jay Wilson. There seems to be many revelations to be made about Diablo 3 at BlizzCon this October. When asked if Diablo 3 could be playable at BlizzCon, Jay Wilson said Blizzard wouldn't let him say. At least a third of the Diablo 3 team are from the old Blizzard North team that worked on Diablo II.
It is not certain whether the next installment of Diablo after Diablo 3 will be an MMO, but he does answer Chris Metzen has plenty of lore and knows what direction the Diablo universe will go after Diablo 3.