Active Voice has some excellent advice on accepting invitations to speak as a member of a panel. I’d add only that his tips are valuable outside the tech world as well.
Panel speaking engagements are tricky – when they’re good and your spokesperson is on his/her game, they deliver a powerful message/image – when they’re bad, they can really affect a program.
It’s important to make sure we place our clients on the right panels and make sure they have a clear role in the discussion. Placing an executive on the wrong panel where he/she has little to say is painful.
Ask to meet the moderator and other panelists ahead of time. Find out who’s in the audience. And if it’s a wrong fit, back out gracefully and wait for the right opportunity.