Well, the "At World's End" title now appears official. The Australian Office of Film and Literature Classification have classified the advertising material for Pirates 3 with an M-equivalent rating.
Their website gives the film the title "Pirates of the Caribbean: At World's End"
Whilst in the US - we met some Disney insiders who had seen footage of the new movie and it is apparently unbelievable - I won't post spoilers, I already know more than I wish I did. This movie is going to be huge!