I heard rumors that mossad warned the NSA that there will be such an attack in time to come, but the NSA didn't take it seriously. But they were just rumors i guess.
Speaking of bin laden, i dont think it was him that came up with the attacks. it was some assosiate of his, who is also part of the al-qaeda netword, one of the big guys, he used to be a policeman in egypt. i've heard people say that he was the mastermind behind the attacks, i think that bin laden was just a financial means of making them take place.
as far as john walker... all i can say is that why he, and all other taliban fighters, would want to join the taliban? i mean, they are the most backwards peolple i've ever seen in my life! there is no religion that dictates such laws as he does does (mohammed omar).