well lets see...we have 2 locations with actual anthrax exposure, and quite a few white powder scares. John Ashcroft just said that "they can target our mail, but we won't stop delivering it." This does not even take into account FedEx, UPS and any other delivery service.
John Ascroft can say what he wants, the real issue is what will the U.S. Postal workers do? I don't think that they will just keep collecting mail and sorting it if they think their life could be endangered. Same is true for UPS, FedEx, etc.
At that point the mail will be quarentined and very carefully examined to determine which has "powder" and what things, fingerprints, etc are in common with the contaimnated mail. Post Office video cameras will be reviewed and perps will be arrested.
The point is that the delivery system for a biological attack will not keep on operating and serving as a delivery mechanism for the biological agents. The terrorist threat delivery system will shut down and stop being a terrorist delivery system very quickly.