Here's what I know, and what I think:
1) If it does happen it won't even start shooting, let alone be written, until 2009, if they want to keep the original production team intact. Gore Verbinski and Jerry Bruckheimer have both said filming the first three was exhausting, and I doubt they'd be ready to jump back in.
2) Keira Knightly won't be in any more of the movies, which likely means they'll consider the story of Elizabeth and Will over.
3) Johnny Depp has said he'll play Jack Sparrow again if the script is good. If they want a good script they need to get Ted and Terry to write it again, since they are the ones most familiar with it. With the current writer's strike, unless they've already started it (which is unlikely, because the news of it being greenlit would have already been announced by now), they won't be starting for some time. Once again, likely around 2009.
4) There's been no word at all if there is another movie in the works. Since this a huge franchise, it is likely that if it had been greenlit, it would have been reported by now. To start speculating on what the plot or who is going to be involved in it is premature at best.

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