It's open to everyone.
To quote from the site:
"The conference theme is “Playing with Boundaries”. The 2007 conference begins with a pre-conference keynote the week of October 8, 2007. The following two weeks, October 15-19 and October 22-26, forty presentations will be posted online to the conference blog for participants to download and view. Live Events in the form of three “Fireside Chats” and a culminating “When Night Falls” event will be announced."
Did I mention that it was free?