Eugene Volokh raises an interesting question: Can the cops drive a Geiger counter down your street? Though almost everyone would agree that the answer is yes, how you get there can be complicated by existing constitutional jurisprudence. Also read the followups at the Volokh blog, here.
My thoughts on the matter: this sort of case is an example of one reason I am an anarchist, not a libertarian. Libertarians try to take a moral framework and construct politics out of it. Most people don't think it is OK to use high-powered microphones to record their neighbors' conversations. The same principle extends to warrantless examination of people's homes for heat (as in Kyllo). But then the same principle extends to warrantless searches for gamma rays. There is no simple dividing line between any of these: each involves energy of some sort that is radiating off of private property. Can you look, or not? From a moral absolutist point of view, looking at such radiation is either OK in all cases, or none.
As a constitutionalist, you (often) don't have to make such hard choices. In this case, the word "unreasonable" in the 4th amendment saves the day. Heat searches: unreasonable. Radiation: reasonable. No problem. Note, though, that it makes a loophole you can drive a truck through. (That's the downside of constitutionalism.)
As an anarchist, you also don't have to make such choices. The law for an anarchist ultimately comes back to consumer choice. If the consumers choose laws which are not morally consistent, that's just fine.