It's not officially 100% confirmed, but this physical display at a Minnesota Best Buy seems like pretty solid evidence. According to a series of photos sent into Joystiq by one of their readers, Best Buy is advertising a midnight launch of Diablo III on February 1st. This meshes with the leaked Blizzard roadmap, which had a Q4 2011 release of the game as a goal, and also makes sense considering the builds of the game I've seen at Blizzard HQ and at BlizzCon in the last few months.
Given that the Diablo games are some of my favorite, this news has left me extremely excited. February 1st will be the day my social life dies, assuming this is true. I've sent an email to Blizzard asking for an official statement, and I'm waiting for a response. Until then, consider this reasonably solid evidence!