much as I hate to agree with the man...
Thomas C. Greene makes a point that had been bouncing around in my head. Which is that, well, since so much secrecy surrounds the source of a lot of the information that the Bush Administration lean on for their terror alerts, should they not also conceal the targets, heck, even the fact that they have the knowledge, the better to pounce on Al Q when the attempts go ahead?

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Presenting details about a potential attack may be enough to deter attempts to make it - terrorists watch the news too.
I'm as keen to want to bash and depose the Bush administration as the next sane guy, but the age of a planned attack doesn't seem to me indicative of its staleness, and I don't consider the current terror alert to be stupid and invalid because it's based on old information. Plans are made (and dispensed with) all the time, Al Qaeda-affiliated groups frequently go years without significant attacks, and the intentions of some cells may or may not affect the intentions of other cells.
What gets my guard up is the timing of the announcement (a Monday, the slow-news vacation season, immediately after the Dem convention and hot on the heels of a potentially hot Bush-favorable news story which ended up not going anywhere) and the content of the announcement (which contains copious non-faint praise for the Maximum Leader). Personally, I expect major Al Qaeda threats to our infrastructure at high-profile locations every other Monday between now and the election. If it happens, that would imply the current warning was created in the context of a political campaign rather than timely terrorist-busting.
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My own attitude to the alert also stems from the timing, but the surrounding circumstances aren't helping (i.e. I am more bothered by the timing than by the three-year-old data)
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And, as far as I can tell, all elevations in alert levels serve to do is increase the number of random, circumstantial and otherwise unwarranted busts of US residents, few of which appear to be relevant to whatever information was key to the alert.