It's been officially announced that Blizzard will launch Diablo III on May 15, 2012. Pre-purchasing is available now via Battle.net.
This will be followed by the release of Heart of the Swarm, the latest installment of StarCraft II, as well as the World of Warcraft expansion Mists of Pandaria.
In order to meet the launch date deadline, Blizzard will omit some features, such as Player vs. Player mode and the Mystic artisan, although PvP is planned as a subsequent patch. In case you've been living in a cave and haven't seen it yet, here's a compilation of all the official trailers: