Bruce Schneier in Wired discusses the challenge that surveillance technology raise for constitutional rights:
Sometime in the near future, a young man is walking around the Washington Monument for 30 minutes. Cameras capture his face, which yields an identity. That identity is queried in a series of commercial databases, producing his travel records, his magazine subscriptions and other personal details. This is all fed into a computerized scoring system, which singles him out as a potential terrorist threat. He is stopped by the police, who open his backpack and find a bag of marijuana. Is the opening of that backpack a legal search as defined by the Constitution?