The news is currently full of articles about the Total Information Awareness (TIA) project that the government is currently embarking upon. The government aims to increase our safety by correlating suspicious activities using multiple data sources. The real danger is that it probably ensures that if the people decide that our government is no longer acting in our interests, they will be unable to do anything about it. And it isn't clear that it will actually increase our safety from terrorists.
At 
salon.com Farhad Manjoo describes how the terrorists used a single credit card to buy most of their plane tickets. The way he describes it, it was intended to seem obvious how this activity should be viewed as suspicious. I think it may seem suspicious only until you look at the larger set of data. Who knows how often this kind of thing already happens? This fall, Tom Hoogendyk and I travelled together to a conference in Chicago. We had put off making our plans until just a couple of days before the conference. I ended up buying his plane ticket for him and, because he was not planning to come directly back (and exactly what he was going to do was somewhat confused) I ended up making three separate transactions for the tickets. Finally, when we got to the airport, we discovered I'd made his reservations for the wrong day. So here you have a young man with no checked baggage, travelling one way, using a ticket purchased with someone else's credit card, travelling on the wrong day. For whatever reason, he was selected for more thorough screening at the airport, but if TIA were in place, I can see him getting dragged off for a lengthy conversation with the FBI and missing the conference altogether.
Ever since I read Edwin Black's IBM and the Holocaust, I've been particularly sensitive to the power of technology to give government power over the people. The data that the Nazis used had largely not been collected with the goal of identifying segments of the population for persecution. It was the new technology that gave the ability of the Nazis to put together disparate, seemingly innocuous, facts to target people for special treatment. Get ready for it to happen again.