Are you as SURE about this as I was about IRAQ having WMD? [Which was 100% by the way]
In any case pre 911 the thought of the TWIN TOWERS being brought down, the Pentagon being hit and the foiled attempts on the Capitol Bldg and WH, there is NOTHING I would not believe AQ being capable of doing.
It doesn't make sense but then again NEITHER did Saddam lying to his own Generals about having WMD.
IMHO we must take ANY threat with a much more believable attitude then we used to.
Oh, I will take the threat super-seriously when it is delivered by someone who is not peddling a book.