The chat will revolve around my TED talk called "An Asteroid Impact Can Ruin Your Whole Day", which I gave here in Boulder in September. You can watch that talk online, which I suggest you do if you want to come to the chat. It's only 14 minutes long, but it does feature me gesticulating a lot.
If you're curious what it will be like, my Discover Magazine co-blogger Sean Carroll did a TED chat in May 2011. This is a fun way to interact with people, and I'm looking forward to it, so drop on by and ask a question!