Hamm himself (Hammself?) has said he's "maybe aged out of" the role. He's certainly shown that he can play Lex Luthor, though. I'm unsure of Hamm as a choice for Superman (he's maybe too ruggedly handsome), but I do enjoy the other idea that has been circling the intertubes: Tina Fey would make an incredible Lois Lane. She's smart, funny, and she doesn't tolerate bullshit. That's Lois Lane, all right.