Diablo III: April 17th. This comes from Italian website mmorpgitalia, which reports that retailers are clearing space for their shelves on the 17th after a tip from Activision Italy.
Blizzard, of course, is declining to comment on the rumor, although Game Director Jay Wilson says that an "all-important announcement" -- most likely the release date -- is imminent. When asked on Twitter how soon the announcement would arrive (on a scale of 1-10), Wilson replied with a simple "9."
Tomorrow begins the CeBIT 2012 trade show in Europe, and Blizzard will be present to show off the Diablo III beta. If an announcement is to be made, this event might make a good a platform as any from which to tell the gaming world which day it's going to need to take off from work.