Lower courts have split on whether agents need to obtain a warrant prior to obtaining real-time or prospective information from cell phone service providers about the cell phone towers used by a targeted subscriber.  Such Cell Site Location Information (“CSLI”) may divulge detailed information about a person’s whereabouts and travels throughout the day, because cell phones may register frequently with nearby cell towers to direct incoming and outgoing calls, text and data.  While courts have analogized between real-time access to CSLI and electronic surveillance, only recently did a Magistrate Judge in Pennsylvania (recognize that access to historical CSLI poses the same risk of abuse as real-time access and requires the same meaningful judicial oversight to satisfy the Fourth Amendment.  (534 F. Supp. 2d 585) She denied, in an opinion joined by three other magistrate judges, the government’s request for historical CSLI without a warrant based on probable cause, and her order was upheld, without opinion, by the District Court. (2008 WL 4191511)  My paper would elaborate on the arguments that I made as an amicus curiae in two briefs: One in the District Court of Pennsylvania in favor of affirming the Magistrate Judge’s decision and one in the Third Circuit in favor of affirming the District Court’s order.  Briefly, the distinction between historical data and real time or prospective data is practically arbitrary, because agents may regularly request records of immediately past use and thereby use “historical” orders effectively to obtain real-time information.  As a substantive matter, methods to obtain historical CSLI may be just as hidden, indiscriminate, and effectively continuous (in that they covers a period of time) as the methods used to Wiretap. CSLI should be subject to a reasonable expectation of privacy (your recent great work supports this) and its acquisition is quite intrusive.  (I have argued elsewhere that The Supreme Court and lower courts have found that the Fourth Amendment requires the highest level of judicial oversight when the government uses a surveillance method that is: hidden, continuous, indiscriminate and intrusive.)  Doctrinally, the beeper cases do not shed much light on the question, but to the extent they do, they support requiring at least a warrant.  The same may be said for the Miller case, which I argue does not support the broad “third-party rule” that is claimed for it and does not support access to historical CSLI on less than a warrant either.  Depending on what happens in the Third Circuit and when, the paper can either discuss the oral argument or the actual decision and assess it against my own views of what the law is and should be.