Way to many people are thinking of a prime RJJ. RJJ hasn't finished a fight since 2002 when he TKO'ed Clinton Woods and hasn't finished a quality opponent since, well you find one. I'm not saying Anderson would win this but I think it's a lot closer then people think. RJJ still has fast hands (though not what they were) and his reactions are still pretty good but his legs are pretty shot. If Anderson stayed outside (does have a 4-5 inch reach advantage) and kept moving around I could see him doing well. He would have a training camp of a couple months to just work his boxing and I would like to see what he could do.
How RJJ does against Calzaghe will tell a lot about what he has left because his fight with Trinidad really didn't.