Rick Rubin shocked me when he said he was producing some of Justin Timberlake's new album. I won't be listening to it though, so not a bother for me.
Either way, if he can somehow help Slipknot churn out a very good metal album when they have two sub-par efforts previously, I think he can give Metallica some words worth hearing. Though I don't think they are "in trouble" either.
-AC
Some insight into what it MIGHT sound like...