Or maybe we feel like its reasonable because there is less chance that something we did in the past might be made illegal (and therefore prosecutable) in the future.

Here is where I tend to disagree with strict constructionists: the framers were particularly aware that future situations might call for adjustments to the Constitution. It was far more important to generally provide a set of principles that could be adopted by a society and then adjusted as their reality evolved.

There is no way that the Framers could have foreseen wall-penetrating radar systems or GPS-enabled cellphones. Our justice system had a hard enough time coming to grips with the telephone and what that meant for Fourth Amendment protections. Since the Right to Privacy isn't enumerated but rather implied by the other Rights, we are probably going to see more cases similar in context U.S. v. Jones where the technical capability to track someone exists that doesn't involve an actual trespass by law-enforcement.

Ok, I follow you. But if the thing that bites us about cell tower logs is the retroactive character of it, what about other technologies that make it easier for the police to surveil us? For instance, the police already have closed circuit camera networks for which they need no court order to monitor people on. In fact, they can even use them to research subjects retroactively.

What about image recognition on those cameras? It won't be long before they can just punch in a license plate number.

There are already problems along these lines. Here in CT there was a minor kerfuffle over the storage of license plate information (gathered by automatic number plate recognition systems on cruisers) long past the stated purpose of identifying stolen vehicles and expired registrations wanted individuals. This fact was actually concealed from the public until there was a full-court press to discover the truth. These kinds of legal questions are no longer theoretical; the feared actions are now happening as predicted.

How comfortable do you feel about the government compiling a dossier of your movements? Do you believe that there isn't some chance that this data will tempt someone to use it improperly?

It feels inevitable, given where technology is heading --- not only in terms of my personal effects emanating trace information about my location, but also just because monitoring and sensing networks are getting so much cheaper.

It also feels (word chosen carefully) like this capability is going to have more positive uses than negative ones. If I want to focus my energy on the law enforcement stuff that really matters, then perhaps I focus on whether we're criminalizing the right things (for instance: drug criminalization is a debacle), and whether we're appropriately constraining the ability of the police to detain and physically search us.

I don't believe that this kind of technology can't be limited in practice and in the law to protect us from the downsides while giving us most of the upside.

We were able to develop a framework to protect telephonic communication (against the wishes of law enforcement I might add) and I think something similar can be accomplished with electronic records compiled by service providers. If the government can pass a law making it illegal for me to sell lemonade without the appropriate permits, they sure as heck can pass a law restricting the sale or distribution of personal data.

I want my location to be made available (to others) to serve ME, but not propagated wantonly willy-nilly to the highest bidder. That's REALLY the major issue here; if these records are permitted to be generated for "legitimate business purposes permitted by law" then it's a free for all, since those records can be subpoenaed. But if we make certain classes of those records "personal identifiable data" then it's easy to erect a barrier to that information legally. The cops and marketers will complain to holy hell about it but I feel like it's a necessary step we as a society need to take to prevent total abuse by the powers-that-be.