The rate at which individuals shed transactional data simply by living in a networked world seems to increase daily. The composite picture of individual activity that can emerge from such data is often of startling clarity, and will likely sharpen with in the future.  We don’t really have a coherent legal theory to address appropriately the growing privacy interests in this kind of data.
The full-scale judicial supervision accorded electronic surveillance and physical searches is probably overkill, and far too cumbersome for data for which basic investigative access is justified. On the other hand, the Miller view that the "consensual" delivery of this data to third parties strips it of any privacy interest looks untenable when one considers the effect of the information aggregated.

I'm disappointed that Rattle was the only one who rememed the Washington Post's recent coverage of National Security Letters, which is being hailed by people on both the left and the right as important journalism.

This commentary provides more of the debate. The main commentator seems to echo the modern conservative view that the legislature needs to be the final arbiter of civil liberties. Forgive me for being tactless, I think in the general case this view is stupid. The whole point of civil liberties is to limit the power of the democratic government with regard to the rights of individuals. Thats what the court system does. The legislature cannot be its own check and balance. The question here is whether this matter rises to the level of a civil rights issue. I think it does.

There is a healthy debate in the threads...

The Volokh Conspiracy - More on National Security Letters: