Two things strike me about the (hopefully) foiled London terrorist plot - and not for the first time. First, technology has enabled small groups of people to be enormously destructive. Second, our traditional notions regarding the balance of civil liberties and protection of the public don't take that into account. We have generally thought that the government may not invade a person's "privacy" (however that may be defined) until it has a justification rooted in a particularized suspicion about that particular person. Once that justification is established, however, substantial encroachments on his or her privacy are permitted. Although the law does recognize lesser invasions for lesser reasons, our rules focus on individualized suspicion and have an "all or nothing" character about them.
The new world may require rethinking this. It may be that lesser invasions of persons' privacy (say computer - as opposed to human - monitoring of cell phone calls) are, under certain circumstances, going to have to be based upon reasonable probability as opposed to individualized probable cause. Civil libertarians are right to suggest that this creates real risk of abuse, but the debate may have to be about how to protect against those abuses, rather than to insist upon 20th century notions of what and when the government can search in a 21st century world.