"Diablo 3" is probably at least a year away from being released, and maybe that extra time is due to the fact that Blizzard is actively working on more than one "Diablo" game! According to an Activision SEC filing, Blizzard is hard at work on "sequels to the 'Diablo' franchise." Here's the pertinent segment, to save you from wading through the massive document:
"Blizzard is currently developing new games, including a new expansion pack to the 'World of Warcraft' franchise, 'Cataclysm,' 'StarCraft II: Wings of Liberty,' which will be released with the new and improved Battle.net, and sequels to the 'Diablo' franchise."
Now, ordinarily I would say that they're just talking about an expansion pack to "Diablo 3," which could, theoretically, be developed alongside the main game. But the fact that the filing specifies "Cataclysm" as an expansion pack and the new "Diablo" games as sequels seems to imply that there are multiple, full "Diablo" releases coming our way.
I reached out to Blizzard about the filing, who probably won't say a whole lot, but I'll let you know if/when they respond.
Worth noting that Blizzard has discussed the future of the "Diablo" franchise (with us no less!) back at BlizzCon 2009. Here's what Jay Wilson had to say about life after "Diablo 3":
"We're not saying this is the end of the 'Diablo' universe, but we are trying to bring this storyline to a close. It's not just 'Diablo 3' — we’ve got plans beyond."