This is from DarkHorizons: Its expected he'll get to work on this once he wraps production on Paramount's "Benjamin Button" adaptation. Fincher's next film "Zodiac" is due out in the Fall.
But from Empire: Before Fincher begins work on Torso, he's expected to go into production on Benjamin Button with Brad Pitt and Cate Blanchett attached to star.
These are all probably guesses but I'm sure well hear something official soon. Keep in mind that Pitt is expecting a baby soon so that might delay proceedings and also that Button will be an expensive film to make. If they delay it by a couple of years they can save a lot on the special effects budget.