From Electronic Frontier Foundation:
New data from law enforcement agencies across the country has confirmed what EFF has long been afraid of: while police are routinely using cell phone location tracking information, only a handful of agencies are bothering to obtain search warrants.
Since 2005, we've been beating the drum loudly, warning that the government's attempts to track a person's physical location through their cell phone requires a search warrant. As we've said again and again, because cell phone tracking can give the government a snapshot of a person's life through their movements, a search warrant is necessary to safeguard against privacy intrusions..