Blizzard had their May Conference call yesterday, and the big news coming from it was that the beta testing phase of Diablo III is scheduled to commence in the third quarter for 2011; that is July 1st - September 30th.
Mike Morhaime also said that they are still pushing to get Diablo III out already this year. But as always, no dates are set in stone, and it is a slim to none chance that Blizzard will bodge their release philosophy "when it's done".
The game has however begun internal testing as of last week which probably means that most of the system and design is set.