That page is no longer displaying as of 1:30 pm PDT. Instead the site seemingly is allowing new registrants to apply.
Microsoft officials declined to say how many attendees they'll be allowing into the conference. (I asked.)
Build 2013 is happening from June 26 to June 28 in San Francisco. In addition to talking about Windows client, Microsoft speakers also will be discussing Windows Azure, Windows Phone, Visual Studio and Xbox at the confab.
Last year, Microsoft's Build 2012 conference sold out in under an hour. There were limited spots available because Microsoft opted to hold last year's event on campus. Many of those who were put on the wait list ended up getting into the 2012 event, however.