Robert Rodriguez talked a little bit about Sin City 2 during his panel last month at Comic-Con International in San Diego. Frank Miller has a script written following the same format as the previous movie, with two entirely new stories. Rodriguez said that the production could kick off this year, and it sounded at the time like that would be the ideal.
Now there's some new talk from Rodriguez, in an interview with We Got This Covered, in which he says that the film's financing is all squared away now, which clears the way for that ideal to become a reality. "We have everything we need so we can just start shooting as soon as we get the pages," he said. "And it's the same thing for Machete 2. We've already got the budget, just waiting for the script. As soon as we're finished writing we get to start the shoot."
I'm not sure what pages he's talking about if Miller has already written the script; perhaps some rewrites are happening. It looks more and more likely though that Sin City 2 will be a real thing sooner rather than later.