No actual date was given on when the WotlK beta will start, but active WoW players who were not in the alpha or already guaranteed a beta slot can take a gamble with the optional-in beta signup that went live recently. It shouldn't be long now. WoW Insider is also reporting on some scuttlebutt from a purported family member of a developer, and their belief that Blizzard is rushing development to meet an arbitrary deadline. With the first phase of beta starting soon there is still plenty of time left for the WoW developers to polish out the 76-80 content. If WotlK is not ready Blizzard will more than likely push the release back as they did with TBC.
[Via WoW Insider]