Actually, I don't think this is anything new...
I think Dean has refused to do this before. He is a "little" man..and he doesn't want his obvious inadequacies to be as obvious as they would be in a "give and take" type setting.
I am sure he is also insisting on being "last"...so he can smear anything that Mehlman says...rather than put his own thoughts out to be disputed.
I am also sure that Russert will accomodate him....cause Fat Timmy isn't any better than Dean...