Were any of the infected letters postmarked prior to 9/11?
I think that will be the ultimate indicator of who is behind this. (Until they ultimatly catch and fry the responsible bastages)
If there was even a single infected envelope mailed prior to 9/11, then you must assume that the Taliban/Bin Ladin/Sadam Hussain crowd is behind the attack in some way.
It would be too much of a coincidence to have two totally separate attacks launched contemporaneously and at targets in the same locals).