It was announced on June 28, 2008, at the 2008 Blizzard Entertainment Worldwide Invitational in Paris, but let`s wait until.
Maybe December. Blizzard likes to release games just in time to get a good Christmas surge, and if its a little hard to find, even better for them. Nothing says "great game" like "sold out"!
Oooohhhh Blizzard, you can't trust them! They always tease us with the schedule released and then postpone it for so long. Can't blame them though, because they always perfected their games before launching it.
I have a friend at Blizzard who says there's absolutely no release date right now, so it'll probably be 2012... That is ages away Starcraft 2 isn't even out yet, and we've been expecting that for 18 months...
You know what Blizzard are like? They announce these games five years in advance. From what I've heard, D3 is pretty close to completion (in terms of game play). Which means, another two years of 'polishing' I actually think Blizzard staff are too busy sitting on their huge mountain of WOW-earned cash to actually do any programming!
I've just heard from my Blizzard friend. Diablo 3 is well underway and, with a good wind behind it, will be out winter 2011/2012. Of course, this could all change, but I'd be very surprised if we don't see it toward the very end of 2011. Still a very long wait, though! I will be giving away some beta tester accounts in the near future! If you like my updates, please do give me some reputation