As I understand, the open debates will invite Bush, Kerry, and some popular independent and third party candidates. If Kerry or Bush do not show I think I heard they will put a straw man in his place.
They should have a 5th grader draw a picture of both of the candidates and then prop up the pictures on the candidate's podiums. Every now and then they'll ask one of the poorly drawn pictures a question and wait 10 seconds for an answer while the camera focuses on it
It wouldn't do much if both Bush and Kerry don't show, but if one shows it could hurt the other.