Here's a very interesting article about terrorism. Here's the very interesting executive summary.
If we assume that A., terrorists hate us and B., they're not stupid, then it logically follows that those terrorists probably are not in the US. The reasoning is pretty straightforward. If the terrorists are intelligent, then they want to engage in terror acts that offer the most exposure for least risk. 9/11 incidents will be few and far between, but random arson, bombings, and other things will be the order of the day. I would find such things to be trivial to pull off. Trained terrorists, therefore, should find it even easier (particularly since we're a wide open country -- this is hardly Israel). Thus, since we tend not to see these things occurring, it suggests that we don't have a bunch of intelligent, trained terrorists operating on US soil.
Mind you, I'm not saying I agree with this idea, but I do find it rather interesting.