I guess the main problem the Feds are facing is the trade-off between just concentrating on 'credible threats,' and the risk of refusing to follow a certain 'ludicrous' threat, and that threat becomes a reality (eg before 9-11 the events that happened that morning would have been classified as 'highly improbable').
Let's just say that the FBI is finding itself under such a punitive work load that i doubt it will even be able to deal with what we can now call 'normal' crime like kidnappings and big-style theft! And with all these 'threat' flying left right and center there is a chance they will be so inundiated with 'leads' that the real one may penetrate their over-loaded dragnets!
And that will lead to another cycle of 'why did you not know?' Saddening!
Osama bin Laden and his pals have demonstrated that they are intelligent and savvy enough to engage in psyops and disinformation campaigns against the American people. I believe they will try to strike when and where it is less, if not least, expected.