There's a lot of new info here:
http://www.scifi.com/sfw/issue414/news.html
Some snippets:
Robert Rodriguez wanted the film images to be as evocative of Miller's drawings as possible. "They're amazingly close," Miller said in an interview. "I mean, we actually had a camera setup that would feature my drawings, and then we'd superimpose the filmic image on top of it, and we'd adjust the shot to match the composition. It's as faithful as anything can ever be."
Miller said that he never intended to make a film version of his dark series, about a crime-ridden city and its unlikely heroes. But Rodriguez's persistence and dedication eventually won him over. "Robert pursued me about it," he said. "He really wanted to make this movie. And he convinced me that technically it was possible, but then he had to convince me that creatively it could be done, because I was still convinced that people would try to slap happy endings on my stories and soften them and focus-group test them and all that nonsense. And it took him quite a while to convince me. I was a tough customer."
Rodriguez said that the violence is stylized in the manner of his earlier films, including Desperado. "I never got any flak for Desperado. ... At a time when people would criticize guys like Quentin [Tarantino] for violence in films for cutting an ear off, off camera, I was mowing down people in my movies and no one ever said anything because of the tone," he said. "And I think that's the same thing for this, that as violent as it is, like in the comic, it felt tempered by the stylization. And that's why we didn't have any trouble with the MPAA [which gave the movie an R rating] or anything, because it was so stylized that they just went, 'This is all right. ... You don't have to cut a frame of it.'"
But Rodriguez agreed that Sin City isn't for children. "Young people shouldn't see it," he said. "It's a rated R movie. ... I'm not making it for that [family] audience. ... I made this an R. I didn't try and trick people into making it a PG-13. ... If parents let their kids in, that's their decision. But that doesn't mean that I'm going to change how we're going to make the movie. Frank made his thing in a vacuum, ... and I wanted to do the exact same thing for cinema and suffer the consequences. If people don't go see it because it's R, that's fine. It's not appealing to the mass audience. It's really just about making the movie we want to make and telling the story that we want to do. "