I don't think this page is intended for public consumption. If you weren't invited or referred by someone you'll most likely be rejected...
Google "ycombinator demo day" and this is hit #3. Sure, someone that wasn't invited/referred isn't going to be allowed to attend Demo Day, but I thought the list of what the companies are doing this round was interesting. So while the event itself might not be for the public, this info at this link doens't seem to be secret.