I don't think Leben possesses the necessary skills to compete at either WW or MW. If there's one thing you can't knock on the UFC, its the WW divison. Leben going down to that class puts him against far superior opponents with guys like St. Pierre, Diego Sanchez, Matt Hughes and BJ Penn.
Realistically, Leben is neither skilled nor athletic enough to really ever pose a serious challenge to the top contenders of the division. Perhaps after more thorough training regiment (I'm talking months of BJJ, Muay Thai, Wrestling), he could be more respectable. Just my two cents.