I was never a huge Supes fan, but I recently read the Infinity Crisis TPB and I have to say, I think you can still do a "dark" superman while retaining a sense of optimism and goodness.
The thing is that studios don't get what it means to be "dark." Yeah, Batman is "dark." But "dark" really just means that the actions in the film have consequences, and that the characters are treated seriously rather than as campy, goofy, etc. Basically, "dark" really just means modern. That's it. Modern.
You could do a modern story of Superman. I thought Routh did a pretty good job with this, actually. I think what needs to come through is that Superman, even in a modern world, still retains his ideals and does what he does because he realizes his own importance. Not in an arrogant, self-satisfied way, but rather in the sense that the world NEEDS a Superman. The world NEEDS a paragon of virtue that they can look to and say "He can do it, why can't I?" Not because he's super-powered but because of his ideals.
In that sense, I think Nolan might actually do a good job. The key, I think, to doing a successful superhero movie is NOT whether it's dark or brightly colored. It's not how many badguys it has, or who the main villain is. It's two simple things: (1) take the material seriously, and (2) recognize that it's not about the superpowers these people have. It's about what drives them to do what they do. Ultimately, I think Nolan gets that about the Batman films. While I could've used a bit more focus ON Batman in TDK, the one thing Nolan absolutely freakin' nailed was Batman's commitment to his mission. He is utterly devoted to keeping Gotham safe, even at the cost of his own life and freedom.
Superman, in that sense, is similar, but goes about his goal differently. But as cornball as his "boyscout" ideals may seem, he is absolutely sincere about them. He believes in being honest. He believes in using his power to help people. He has a true sense of DUTY about what it is he does. Ultimately, Superman does what he does because he's the ONLY person who can do it. He wears his costume because he is a symbol. He is a living embodiment of what is good in this world and the power of human determination. If you want to make a good Superman movie, you focus on THOSE things. You show that he never gives up, that he's utterly committed, and that he stands for something. Something that doesn't have to be treated as corny or cheesy. Whereas Batman exists to inspire fear in those who are evil and act as a living sentinel for the people of Gotham, Superman exists to inspire humanity to strive to be better and to remind them that even in a world full of pain and bleakness, there's still goodness and that it does win in the end.