From the EFF Deeplinks blog, in the matter of US v. Warshak:
As the Court held today,
Given the fundamental similarities between email and traditional forms of communication [like postal mail and telephone calls], it would defy common sense to afford emails lesser Fourth Amendment protection…. It follows that email requires strong protection under the Fourth Amendment; otherwise the Fourth Amendment would prove an ineffective guardian of private communication, an essential purpose it has long been recognized to serve…. [T]he police may not storm the post office and intercept a letter, and they are likewise forbidden from using the phone system to make a clandestine recording of a telephone call–unless they get a warrant, that is. It only stands to reason that, if government agents compel an ISP to surrender the contents of a subscriber’s emails, those agents have thereby conducted a Fourth Amendment search, which necessitates compliance with the warrant requirement….
“Today’s decision is the only federal appellate decision currently on the books that squarely rules on this critically important privacy issue, an issue made all the more important by the fact that current federal law–in particular, the Stored Communications Act–allows the government to secretly obtain emails without a warrant in many situations. We hope that this ruling will spur Congress to update that law as EFF and its partners in the Digital Due Process coalition have urged, so that when the government secretly demands someone’s email without probable cause, the email provider can confidently say: “Come back with a warrant.””
PDF download of the decision. Big victory for online privacy advocates!