Probabilistic: "[A] reasonable expectation of privacy exists when a sensible person would predict that he will maintain his privacy."It's an interesting and thought provoking break down. Of course, it suffers from the same problems as any attempt to explain 4th Amendment jurisprudence. The reasoning in cases often bleed over into one another and as you read his examples offered as proof you can often think that it would better fit as proof of something else. Still, as I step back from the minutiae and look to the general model offered I only really have one critique.
Private Facts: Is "the information obtained  sufficiently private, revealing, and worthy enough for its disclosure to trigger Fourth Amendment protection?"
Positive Law: "[A]n expectation of privacy becomes reasonable when individuals must violate the law to defeat it."
Policy: "Normative policy preferences govern the reasonable expectation of privacy inquiry."
I don't think that the "Private Facts" model stands up under scrutiny. I think it is better suited as a subsection of the "Policy" model. Or, perhaps I should say that I think there are two policy questions in every 4th Amendment analysis. There is the pre-examination "Legitimate Expectation" policy question and the post-examination "Balancing Test" policy question. The overall model I envision looks something like this:
In this model the Legitimate Expectation policy test is the gatekeeper of the 4th Amendment. If your alleged violation isn't a legitimate one it merits no further examination. The examples which come to mind are prisoner searches in prisons and dog sniffs. However, if your expectation is "legitimate" it then merits closer examination as to its reasonableness.
Before determining whether there is a reasonable expectation of privacy there is some initial test which would determine whether initial analysis will be probabilistic or under under positive law. While in the vast majority of cases the two analyses would overlap there are any number of times they will not. We know from the case Orin cites, California v. Greenwood, that the fact something is illegal does not make it normatively unconstitutional and we can infer from Atwater v. Lago Vista (although it is a seizure case) that the fact some act exceeds normative expectations does not always matter if it is legal. However, I don't know what the test is.
Once run through the probabilistic or positive law sorters the act contested is tentatively tagged as exceeding or falling within the bounds of whichever constitutional test was applied. It then moves on to the Balancing Test policy examination. Many, if not most, of the contested acts should breeze right through that section because their effect will be relatively minor. Those acts exceeding the initial constitutional tests will cause evidence to be excluded. The other acts will result in constitutionally admissible evidence.
Nevertheless, there will always be those cases which would result either in a major change in the way law enforcement operates or a major change in the understood constitutional rights of citizens. In such cases the Court must engage in a more serious examination of the expected end result. On one hand, one would hope that the Court would weigh things more heavily in favor of liberty. One the other hand, I agree with Justice Jackson that it is not the Court's job to turn the Constitution into a suicide pact through unwavering, fanatical 4th Amendment absolutism. See Terminello v. Chicago. During this point an act which is putatively unconstitutional under the probabilistic or positive law tests may flip to constitutionality if it is the only practical way to stop the next 50 major terrorist attacks. As well, an act which is putatively constitutional may fail if it would result in 1 out of every 10 random citizens having their houses searched every month.
So that's the overall model which I threw together in reaction to Professor Kerr's Article. I don't have time to do a critique of the second part or even to engage in much discussion of what I've put together (I've got a couple of real nasty jury trials coming up) so I'm going to ask you all to send your constructive critiques to Orin's original post over at The Volokh Conspiracy.