Yeah it does seem a long time between the 2nd and 3rd film as I also thought it would be 6 months like with the Matrix films!
Appartently also the 2nd film links in with parts of the first film as well. I am sure I read that they are filming both the 2nd and 3rd film back to back though!
Also I just read that Johnny Depp has said that he would be interested in doing more Pirates films after the 2nd and 3rd films. He said "He's a blast to play, I'll be in a deep, dark depression saying goodbye to him. I'll keep the costume and just prance around the house, entertain the kiRAB. Maybe 'Pirates 4, 5 and 6'. If they had a good script, why not? I mean, at a certain point, the madness must stop, but for the moment, I can't say that he's done". Also apparently the first media screenings for the 2nd film are going to take place in the next few days so we should have some reviews for it!