Clauset and Young, in their new paper http://www.arxiv.org/PS_cache/physics/pdf/0502/0502014.pdf [Broken], apply standard modern statistics to a database of terrorist strikes ordered by the number of people killed or injured. They find that extreme cases, such as 9/11, are not outliers, but find their place within a scale free power distribution with exponent near 2 (may actually be nearer 1.8). From this they conclude another strike with as many casualties or more than 9/11 can be expected within the next 7 years. They remark that the ability to kill people has been steadily increasing, which may underly this scale free behavior. I note that scale free or "critical" behavior in many systems is associated with the approach to a change of state.