As was recently reported, helmer Gore Verbinski has committed to direct a Pirates sequel, so it's likely that he'd be asked to take the reigns of the third film if they're shot back-to-back. The first film's principal cast (Johnny Depp, Orlando Bloom and Keira Knightley) have all agreed to return for one sequel and the studio is reportedly entering into negotiations with them for an additional sequel that may be filmed in quick succession.
Screenwriters Ted Elliot and Terry Rossio (The Mask of Zorro, Shrek) are currently working on the script for first sequel. It's widely known that the pair have already conceived a third movie. While promoting the first film Elliot told the press, "I have two more stories to tell about Jack Sparrow."
We'll keep ya posted on the latest, matey!