The call for beta testers did go as planned with a late-March timeline, but with only four days left, it doesn't seem like Valve is going to make their second deadline. Over the weekend, Valve put this message up on their Steam for Mac beta site: "Sorry! This stage of the Steam on Mac beta signup is now closed. Applicants will be chosen in the coming weeks. If we open the beta to a larger audience we will let everyone know!"
We've sent an email to Valve asking if they have a new public release date planned, but have yet to hear back from them. As soon as we do, I'll let you know!