I don't know if it's mainstream news everywhere yet, but here in Holland I just read on the website of the Telegraaf that US authorities were aware of the risks in the case of Faruk Abdulmuttalab, the socalled underwear bomber. They did however choose to not deny him the right to board his flight to Detroit, because they thought that might make their task of infiltrating in Al Queda (I suppose in Yemen) more difficult.
Now, I do understand that sometimes you have to make very difficult choices when it comes to weighing interests. However, I personally think that the decision is pretty easy if you have reached the point that the 'few' becomes a a planeload of people plus possible victims on the ground.
If these reports turn out to be true, then it's really time for some stiff evaluations of policy in the dept of Homeland Security.