The Word of Notch has word that Minecraft has entered beta testing today, as promised. The update has details on what's new since the game left the alpha stage, and a subsequent update has word on a new version 1.0_01 patch that's now live, and promising another patch tomorrow to address these issues: "There’s a serious problem getting the client started for some people (especially Mac OSX?). I’ve also heard about weird item duplication bugs with dropped items, and a severe performance reduction."