But you'd be wrong!

First, terrorists are (gasp) on occasion literate (ed.: shurely not!) and have been known to keep abreast of the news. One can presume that, if it's been reported in such mainstream media sources, the presumptive terrorists (assuming they actually exist and are in fact planning to hit these targets) now know that the steely eye of The Man is upon them and their prospective targets. A rational terrorist would of course conclude that it is better to select another, unexpected, target, and strike there. A non-stupid rational terrorist would have planned this out in advance and have a wide range of available targets, only some of which his minions would know about, in order to maintain all that good stuff like operational secrecy. So now, by opening your mouth and warning the public, you've managed to trade off a possible attack against a known site (or set of known sites) for a possible attack against pretty much any site but those. (Although I suppose if you're up against a really devious terrorist, he'd conclude that because you've done this analysis, you'd think that he'd attack anywhere but there, and thus he should attack precisely there. But as you are not a great fool, you would have counted on it, etc.)

Second, you've just given the cell (assuming they exist, yadda yadda yadda) the heads-up that you know they exist and what they're planning, and thus give them a wonderful chance to go into hiding, call off their plans, and live to strike another day. When you won't know about it. Just peachy!

Third, if the cell exists, they now know how you know about them and can make plans to change their means of communication (c.f. Times):

Saturday's warnings appeared to be linked to the arrest on July 19 in Texas of Farida Goolam Mohamed Ahmed after she entered the United States from Mexico by crossing the Rio Grande and crawling through the brush.

According to several news accounts, she had an altered passport along with several thousand dollars in cash and an airline ticket to New York. CNN reported that she was charged with illegal entry, making false statements and falsifying a passport.

The new information was first reported last night by ABC News, which said it had learned from several law enforcement agencies that an overseas source, which the network did not name, had provided information about suicide attacks being planned by Al Qaeda in the city. The ABC report said intelligence sources had described a plan by Al Qaeda to move non-Arab terrorists across the Mexican border into the United States. She has admitted to no criminal intent.

Another federal law enforcement official said the woman was believed to have been on a terrorist watch list. He said she might have been sent as "a courier" to pass along either a message or documentation to someone in the United States.

So, in information-theoretic terms, you've just given away a piece of information you have so that it can now do you no good, without (as far as anyone can tell) having got anything in return. Gee, isn't that smart?

If you want to actually engage in protection, a better course of action would be to keep quiet, but increase surveillance / security around the target sites. Continue trying to discover who the group is (since they now don't know that you know about them), and try to flip the courier (probably not possible, but it doesn't hurt to try). Ordinary citizens won't be able to do anything useful to protect themselves except not go to the targets anyways, so there's not much you can do on that front. As Will might put it, At least we'll die with harness on our back..

As a means of trying to protect the public, what Secretary Ridge has done is not only utterly useless, but rather counterproductive. It's even worse than asking for photo ID at airports and so on, which while also utterly useless, is at least not directly harmful (except inasmuch as it diverts attention from even more egregious security holes). Which reminds me to blog about that sometime....