I just finished watching a grainy videotape of a BBC documentary, The Power of Nightmares. It's an excellent documentary tracking the parallel rise of the neoconservative movement and violent Islamic fundamentalism, but good luck seeing it here in the US, where distribution appears nearly impossible.
Towards the end, they discuss how the Brits and later the Americans adopted the precautionary principle used in the environmental movement and applied it to potential terrorism and the need to limit civil rights. Any potential terrorism one can imagine must be prevented, and civil rights be damned.
This misuses a useful concept, although to be fair, some environmentalists are guilty of the same. Saying something should be judged dangerously risky and prevented until proven harmless is wrong, whether it concerns environmental or "terror" issues. A much better use of the principle would be to say that once you have some actual evidence of a risk - something more than imagination but not necessarily conclusive - then you have to balance the risks against the benefits. And as for negating the concept that any risky technology must be allowed unless proven harmful, the precautionary principle is great.
And of course, when civil rights are involved, it's not just about maximizing utility in a situation of limited information.