Prosecutors don't need a warrant to obtain a cell phone's location.

Prosecutors do not need a warrant to compel a cellular phone service provider to turn over data about call location, a federal judge in Washington said in a ruling unsealed Wednesday.

The ruling examines the government’s attempt to get data from the undisclosed service provider amid a U.S. Attorney’s Office investigation of an armed robbery of an armored truck.
Ruling: http://legaltimes.typepad.com/files/lamberth_ruling.pdf
Chief Judge Royce Lamberth of U.S. District Court for the District of Columbia redacted the name of the service provider, the target phone number and the name of its alleged user.
Lamberth ruled in part for prosecutors, reviving the government’s push to obtain cell phone data. The judge reversed a magistrate judge’s ruling from August.But Lamberth did not rubberstamp the government’s request, submitted under the Stored Communications Act. Instead, he said prosecutors must present additional evidence to prove the requested data is material to the armed robbery investigation. The burden is lower than the one a warrant would require.
The dispute gave the court the opportunity to explore the scope of a controversial Washington federal appeals court ruling about the propriety of warrantless GPS surveillance.http://legaltimes.typepad.com/blt/2011/10/judge-no-warrant-needed-for-cell-phone-location-data-.html