In a recent Q&A session with Frank Miller touched on Sin City 2. Here's what the director had to say on the topic:
I can't talk about my projects, because I don't believe a movie is real until I see the title on the screen. There are many things that can go wrong. But I can tell you that I'm very close to begin 'Sin City 2' with Robert Rodriguez. We have to arrange a few things and we'll be back in action.
Would you see Sin City 2?