Mathematician John Allen Paulos (author of Innumeracy) has the numbers that show why Total Information Awareness won’t work, and will have a tremendous cost in both money and freedom.

…the system will arrest almost 3 million innocent people, about 3,000 times the number of guilty ones. And that occurs, remember, only because we’re assuming the system has these amazing powers of discernment. If its powers are anything like our present miserable predictive capacities, an even greater percentage of those arrested will be innocent.