In an earlier blog post on a pending case in the Sixth Circuit, Warshak v. United States, I figured there was no way the court would get to the merits of the Fourth Amendment issue lurking in the case: there were no facts yet and no decided statutory law, and surely the panel wouldn't be so reckless as to presumptively strike down a federal statute in the absence of facts or law given the procedural problems with the case. I had a funny feeling things would turn out differently when I learned who was on the panel, though, and that funny feeling turned out to be justified: the panel just issued a blockbuster decision that tries to answer how the Fourth Amendment applies to e-mail (all without any facts, amazingly) based on arguments from amicus briefs that the government didn't address all in the context of an appeal from a preliminary injunction. Wow. More on the decision later today.

  UPDATE: Here's the key part of the opinion:

[W]e have little difficulty agreeing with the district court that individuals maintain a reasonable expectation of privacy in e-mails that are stored with, or sent or received through, a commercial ISP. The content of e-mail is something that the user "seeks to preserve as private," and therefore "may be constitutionally protected." Katz, 389 U.S. at 351. It goes without saying that like the telephone earlier in our history, e-mail is an ever-increasing mode of private communication, and protecting shared communications through this medium is as important to Fourth Amendment principles today as protecting telephone conversations has been in the past. See Katz, 389 U.S. at 352 ("To read the Constitution more narrowly is to ignore the vital role that the public telephone has come to play in private communication.")

Notably, the court's Fourth Amendent analysis combines aspects of the probabilistic, private facts, positive law and policy model (the above-quoted section being from the policy model section).