A few days ago I mentioned the possibility that a President Gore could have prevented the 9/11 attacks. Of course, I have no idea if that's true. Gore himself seemed to suggest as much based on the Clinton/Gore administration's successful disrupting of the millennium bomb plot and on Bush's apparent dismissal of the "Bin Laden Determined to Attack U.S." memo. But this is all counterfactual and, by definition, unknowable.
Malcolm Gladwell has a wonderful article (which also appears in his new book) on our bad habit of Tuesday-morning-quarterbacking terror attacks. As he notes, if you look at the stream of clues leading to the 9/11 attacks, to the bombing of the U.S.S. Cole, to the invasion of Israel at the beginning of the Yom Kippur War, etc., it seems incredibly obvious. You wonder what sort of idiots couldn't put these pieces together. But, as Gladwell points out, the reports and articles describing these clues inevitably leave out the false positives -- the clues that led nowhere.
For example, prior to the Yom Kippur War, Israeli intelligence had been tipped off about numerous invasions from Egyptian forces over the previous two years. All those warnings had amounted to nothing. Besides, preparing for invasion is itself a very costly activity, and can help precipitate a war. In another example, Gladwell notes that the FBI had received letters in 1998 indicating that an attack on the U.S. Embassy in Nairobi was imminent. These were true, but they were received along with 68,000 other warning letters between 1995 and 1998, the vast majority of which were false.
The problem here is not necessarily obtaining the right information -- often, we already have it. The problem is that we have so much wrong information, or at least that we don't always know how to distinguish between right and wrong information. Those who criticize the current administration for failing to pick up on the obvious signals prior to the failed airborne attack over Christmas are often arguing for obtaining ever more bad information. Screening all young Muslim men, for example (leaving aside the moral implications for the moment), would require TSA officials to spend lots more time dealing with lots more innocent people. It would increase the noise level, only making it harder to detect the signal.