From the Electronic Frontier Foundation:
San Francisco - In an unprecedented victory for cell phone privacy, a federal court has affirmed that cell phone location information stored by a mobile phone provider is protected by the Fourth Amendment and that the government must obtain a warrant based on probable cause before seizing such records.
The Department of Justice (DOJ) had asked the federal court in the Western District of Pennsylvania to overturn a magistrate judge's decision requiring the government to obtain a warrant for stored location data, arguing that the government could obtain such information without probable cause. The Electronic Frontier Foundation (EFF), at the invitation of the court, filed a friend-of-the-court brief opposing the government's appeal and arguing that the magistrate was correct to require a warrant. Wednesday, the court agreed with EFF and issued an order affirming the magistrate's decision.
EFF has successfully argued before other courts that the government needs a warrant before it can track a cell phone's location in real-time. However, this is the first known case where a court has found that the government must also obtain a warrant when obtaining stored records about a cell phone's location from the mobile phone provider.
My commentary about specific extension of the Fourth Amendment to electronic documents can be found here.