Is Pre-Emptive Self-Defense Justifiable?
Let's further simplify the question: if X has knowledge that Y seeks to harm its person or property, is X justified in initiating force against Y? It must be said that it is justified for X to repel the invasion of Y once such an invasion of person or property has been initiated. This is the only justifiable definition of "pre-emptive self-defense" in my view. Smacking Y in the face because X thinks Y is going to steal his Twinkie does not rise to the level of justification of the use of force.
Further, and especially in the case of 9-11 and Osama bin Laden, justified use of force by the United States is limited to acts of aggression committed against the specific individuals responsible. X is not justified in killing Z in pursuit of Y. In fact, this makes X a criminal himself. Roderick Long has a post here that touches on this subject.