The Panels program at SIGGRAPH 2004 is different from past years, and far more accessible. All it takes to be considered is to submit either a one-page panel topic idea or later (once the topics are chosen) to submit a one-page position statement.
The panels topics deadline is coming up this week (November 12th), so go check it out at http://www.siggraph.org/s2004/cfp/panels/. Don't wait and expect there to be topic you are interested in, you can help set the agenda for SIGGRAPH 2004 right now!