From FourthAmendment.com:
An FBI Agent trolling in a Yahoo! child pornography chat room was able to tell that “markie_zkidluv6” had uploaded child porn. An administrative subpoena was served on Yahoo! for the subscriber information, and information was used to get a search warrant for Bynum’s a/k/a “markie_zkidluv6″‘s house. There manifestly is no reasonable expectation of privacy in one’s subscriber information with an internet service provider.
From the opinion:
Bynum voluntarily conveyed all this information to his internet and phone companies. In so doing, Bynum “assumed the risk that th[os]e compan[ies] would reveal [that information] to police.” Smith v. Maryland, 442 U.S. 735, 744 (1979). Moreover, Bynum deliberately chose a screen name derived from his first name, compare “markie_zkidluv6” with “Marques,” and voluntarily posted his photo, location, sex, and age on his Yahoo profile page.
Even if Bynum could show that he had a subjective expectation of privacy in his subscriber information, such an expectation would not be objectively reasonable. Indeed, “[e]very federal court to address this issue has held that subscriber information provided to an internet provider is not protected by the Fourth Amendment’s privacy expectation.”
The case is United States v. Bynum, 08-4207 (4th Cir. May 5, 2010). Read more about it on FourthAmendment.com.