Not only that, the court extended the reasoning to emails as well. Here's the main quote from the opinion:
Here, we must first answer the threshold question: Do users of text messaging services such as those provided by Arch Wireless have a reasonable expectation of privacy in their text messages stored on the service provider's network? We hold that they do.This will open up all sorts of defenses for criminal lawyers. It's a safe bet that the mayor of Detroit wishes this case covered Detroit instead of just the Ninth Circuit.
In Katz v. United States, 389 U.S. 347 (1967), the government placed an electronic listening device on a public telephone booth, which allowed the government to listen to the telephone user's conversation…The Supreme Court held that listening to the conversation through the electronic device violated the user's reasonable expectation of privacy…In so holding, the Court reasoned, "One who occupies [a phone booth], shuts the door behind him, and pays the toll that permits him to place a call is surely entitled to assume that the words he utters into the mouthpiece will not be broadcast to the world. To read the Constitution more narrowly is to ignore the vital role that the public telephone has come to play in private communication"…Therefore, "[t]he Government's activities in electronically listening to and recording the petitioner's words violated the privacy upon which he justifiably relied while using the telephone booth and thus constituted a 'search and seizure' within the meaning of the Fourth Amendment."
