AQ's strategy here is sound: Since they cannot hope to beat us in the field, they must fight us on another terrain, one where our military superiority counts for nothing the information battlefield. Just as the terrorists of 11 September scored a major victory by using our own commercial passenger aircraft as cruise missiles, the AQs intend to use our omnipresent and instantaneous electronic news media as a psychological warfare weapon, again turning our own strengths against us. By using rumors, threats, and propaganda to destroy morale and weaken the economy, the AQ seeks to undermine our willingness to wage war while at the same time causing economic damage and all without any risk to themselves.
(Ironically, an actual, physical attack on a U.S. city resulting in significant loss of life would almost certainly doom AQ; if a U.S. city were to be destroyed or seriously contaminated, the political pressure on the President for a retaliatory action of the most severe kind against a broad set of Islamic targets would be almost irresistable.)
OBL and company have learned the lesson taught by Ho Chi Minh well: destroy American support for the war on the home front, destroy the public's sense of security, and destroy Americans' trust in their government, and you have won the war. Cynicism is the terrorists cyanide; panic is their atomic bomb. Don't give in to either! Make your support of the war effort known, and do your utmost to fight against the cynics who say the war is pointless or unwinnable. When terrorist threats are rumored, don't spread the rumors; instead, gather all the information you can from reliable sources and consider the seriousness of these threats in a calm, sober, and rational manner.
Obviously, we should make every effort to protect ourselves from physical attack, but before we evacuate the cities, quit our jobs, dump our investments, or convert our savings to gold, lets each make sure we have the facts.
But who really knows about the threats among the general population? I doubt that Dean's guy in the truck with the confederate flag knows or cares.