Law enforcement is tracking Americans' cell phones in real time-without the benefit of a warrant. Amid all the furor over the Bush administration's warrantless wiretapping program a few years ago, a mini-revolt was brewing over another type of federal snooping that was getting no public attention at all. Federal prosecutors were seeking what seemed to be unusually sensitive records: internal data from telecommunications companies that showed the locations of their customers' cell phones-sometimes in real time, sometimes after the fact. Prosecutors were using the cell phone as a surreptitious tracking device," said Stephen W. Smith, a federal magistrate in Houston. "And I started asking the U.S. Attorney's Office, 'What is the legal authority for this? What is the legal standard for getting this information?'" Those questions are now at the core of a constitutional clash between President Obama's Justice Department and civil libertarians alarmed by what they see as the government's relentless intrusion into the private lives of citizens. There are numerous other fronts in the privacy wars-about the content of e-mails, for instance, and access to bank records and credit-card transactions. The Feds now can quietly get all that information. But cell-phone tracking is among the more unsettling forms of government surveillance, conjuring up Orwellian images of Big Brother secretly following your movements through the small device in your pocket.