Orin Kerr at The Volokh Conspiracy reports:
No Fourth Amendment Protection in E-Mail Addresses, IP Addresses, Ninth Circuit Holds: Commentators and Congress have long assumed that government surveillance of non-content “header” information like e-mail addresses and IP addresses, typically done by a service provider, do not violate a Fourth Amendment “reasonable expectation of privacy.” Today the Ninth Circuit became the first court to hold this directly in United States v. Forrester.
You can read the opinion here. From the opinion:
Alba challenges the validity of computer surveillance that enabled the government to learn the to/from addresses of his e-mail messages, the Internet protocol (“IP”) addresses of the websites that he visited and the total volume of information transmitted to or from his account. We conclude that this surveillance was analogous to the use of a pen register that the Supreme Court held in Smith v. Maryland, 442 U.S. 735 (1979), did not constitute a search for Fourth Amendment purposes.
(Via How Appealing)