I agree, you don't know; on the one hand, you risk "looking foolish" by blaming things on terrorists (or somebody), OTOH, you risk "being foolish" by ignoring everything or believing it's all naturally occurring or coincidences. There seems to be no position to take, on any one event any more, where you look like a paranoid or an ostrich, to different people.
That's not to say that the cruise ship and military incidents are definitely terrorism, but it is a mistake, in this day and age, to immediately try to explain it away. We can't afford to assume the hypothesis that these incidents have a benign cause. THAT is likely why President Bush is ordering the Smallpox vaccines for troops. He has the burden of making that decision you described, and unlike his predecessor, he is erring on the side of caution.