Producer Stephen L’Heureux spoke with Comics2film and shared some info regarding SIN CITY 2:

The second film will not be based on some of the remaining SIN CITY comics, but will be an original story by Frank Miller. Miller will once again co-direct with Robert Rodriguez.

Production will commence in the second half of 2010.

I thought the first was great, pulpy fun and can’t wait for the second visit to Sin City.