It seems that Blizzard had intended to reveal the release date for Diablo III at its BlizzCon event, but for whatever reason, didn’t. This was revealed by an 17th August, 2011 email from a blog that accurately leaks information from videogame companies.
The email accurately predicted the announcement of the World of Warcraft expansion Mists of Pandaria, including the name of the expansion, the panda race and its perks, the new class, the level cap, and so on. The email also accurately leaked the Starcraft II units.
The one thing that didn’t occur was the announcement of the Diablo III release date, which the email claimed, ”Diablo 3 Release Date will be announced, Dates are tentative: November 24, 2011, Backup Q1 2012: Jan 17, 2012”
Analysis: Assuming that at the time Blizzard was fully intending to announce a 24th November or 17th January release date, it seems logical that the latter would have been the date announced at the convention.
There are now two possibilities: either the game will be released on 17th January, or the reason that it wasn’t announced is that Blizzard realized, undoubtedly to their dismay, that Diablo III would have not been ready by that date either.