Seems there wasn't just the 6 August PDB that notified the Bush Administration of the upcoming 9/11 attack. Secretary of State (Then National Securiy Adviser) Rice, Secretary of Defense Rumsfeld and Attorney General Ashcroft were all briefed two months beforehand. Interestingly, the then-current Secretary of State, Powell, was not in the briefing. Also, it's very interesting to note that the 9/11 Commission staff director Philip Zelikov, who was an NSC Deputy serving directly under Rice at the time, "...didn't respond to e-mail and telephone queries from McClatchy Newspapers" concerning why the 9/11 Commission never included this meeting in their report.
It was relatively easy to dismiss the PDB as a fluke. One warning, delivered to one person, a person not really known for being all that bright. But to have the same warning delivered to three principal officials well before the event is a bit harder to explain away. As DailyKos explains, the evidence that at least Rice was fully aware of the upcoming attack is damningly specific.
Of course, what Rice, Rumsfeld or Ashcroft DID about the warning remains an open question.