If the police arrest you, do they need a warrant to rifle through your cellphone? Courts have been split on the question. Last week the Obama administration asked the Supreme Court to resolve the issue and rule that the Fourth Amendment allows warrantless cellphone searches. In 2007, the police arrested a Massachusetts man who appeared to be selling crack cocaine from his car. The cops seized his cellphone and noticed that it was receiving calls from “My House.” They opened the phone to determine the number for “My House.” That led them to the man’s home, where the police found...