I don't know, it seems like a pretty standard terrorist mind-f*** to me. Bin Laden issues vague warnings about "storms of airplanes" and "stay away from tall buildings and aircraft" in his video tape, then the authorities get more specific threats, via anthrax-laced mail bombs sent to journalists, that are a bit more specific--then they are left with the problem, do we tell the public? Do we ban all trucks from crossing that bridge? Are they talking about the bridge in FL or the one in Chicago? What is the timeframe? Is this a head fake? Will we look bad if we don't publicize the threat and then they hit it? Etc., etc. This is just how terrorism works.
"Hey Mohammed, whaddya say we screw with their minds and send a bunch of people envelopes with cryptic, ambiguously worded threats!"
"Good idea, Ahmed! Even better, we put talcum powder or sugar in some of them, and anthrax in others!"
"I like it! Allah would be proud!"
"Ooh, ooh, let's REALLY screw with their minds and make ominous references to OTHER bombing incidents, like that Oklahoma City thing!"
"Mohammed, you are one cold sumb**ch!"