I think Routh is out, I truly do. At the end of the day, he is heavily tied to "Superman Returns" and that film is considered by the majority to be medicore at best. It was a financial failure, though that would have been near impossible to avoid. WB will not likely want to risk their "reboot" on him. If people see him, no matter who else is cast in the film, or how much they say in commercials "from the creative team who brought you The Dark Knight trilogy" (as it will be by then) people will presume it's "Superman Returns 2" -- it's just too big a downer-risk to have him.
Welling, I see as a dark horse long shot. He has two things working for him; the "success" of Smallville, and that he is Superman to most people under 25. He has the baggage of Smallville against him though, and WB is likely not keen on bringing it with them. If he got it? It would be a clean break. We would see nothing at all attached from Smallville. Not one actor, plot line, nothing. It would be Tom Welling playing a different Clark Kent/Superman. They would make sure people knew this by casting someone who would be talked about as Lois and as the villain(s) in the film. If they where smart they would keep Lex as far away as possible.
I do see Hamm as a realitic lead contender. But I also see him as just a touch, barely, too old (by Hollywood standards) and he looks his age. He's 39 right now, which makes him 41 at least for this film. That does not give him many sequel oppurtunities in the role before he's "too old" by Hollywood standards. It's a risk to cast him, only to have him hold the role once or twice before they have to look elsewhere. By comparison, Christian Bale by comparison is only 35/6 right now, was 30 when he did "Batman Begins" and played that exact age - and 9 years its junior as a college-aged Bruce for part.
Going over those three choices, trying not to be bias, Welling at 33 seems ideal. He has the youth backing from his time on Smallville, he's established in the role, it suits him well, but most important: his age is just right. Smallville will have been over 2 years by the films release; he'd be 34/5 filming it. He looks younger though, always has, so he's got an easy dozen-plus years further to play the role before looking "too old" by Hollywood standards.
If it where between those 3 choices only, the arguments I just made would be many of the ones made by the studio. Granted, at this point, it's Chris and Jonah Nolan's call. Chris Nolan has WB eating out of the palm of his hand after TDK, and everyone knows it -- especially the studio, or they'd never fathom entrusting a rookie like Jonah to directing a Superman reboot-ish picture. Whoever they want, I expect they'll get. In the wake TDK, there are few folks who would refuse Nolan's request to be in a Superhero flick I think; you don't balk at the man who delivered a billion-dollar superhero film and a posthumus Oscar win for its lead villain. You just don't.
If I where in charge? I'd put Welling and Hamm in the damn outfit with the hair and have them read some old Reeve lines and if that didn't work, flip a coin. LOL.
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Zachary Levi...Brandon Routh... 
