Excellent column by Bruce Schneier:
A future in which privacy would face constant assault was so alien to the framers of the Constitution that it never occurred to them to call out privacy as an explicit right. Privacy was inherent to the nobility of their being and their cause. Of course being watched in your own home was unreasonable. Watching at all was an act so unseemly as to be inconceivable among gentlemen in their day. You watched convicted criminals, not free citizens. You ruled your own home. It's intrinsic to the concept of liberty.
Too many wrongly characterize the debate as "security versus privacy." The real choice is liberty versus control. Tyranny, whether it arises under threat of foreign physical attack or under constant domestic authoritative scrutiny, is still tyranny. Liberty requires security without intrusion, security plus privacy. Widespread police surveillance is the very definition of a police state. And that's why we should champion privacy even when we have nothing to hide.