In the name of keeping us safe, the federal government argues it has a right to know the location of our cell phones:

...the Obama administration has argued that warrantless tracking is permitted because Americans enjoy no "reasonable expectation of privacy" in their--or at least their cell phones'--whereabouts. U.S. Department of Justice lawyers say that "a customer's Fourth Amendment rights are not violated when the phone company reveals to the government its own records" that show where a mobile device placed and received calls.

I disagree.  Of the two types of wireless tracking, federal government collection of prospective data is the most alarming:

It comes in two forms: police obtaining retrospective data kept by mobile providers for their own billing purposes that may not be very detailed, or prospective data that reveals the minute-by-minute location of a handset or mobile device. 

I do believe, harkening back to Justice Harlan's concurrence in Katz, that we have a reasonable expectation of privacy, both subjectively and objectively, in the present location of our cell phones.  I would place the hurdle of a search warrant requirement in the way.