From the Center for Democracy and Technology:
"A federal court ruled September 10th that stored cell phone location information is protected by the Fourth Amendment. The court said the government needed a warrant, based on probable cause, in order to gain access to stored cell phone location information. Other courts have required probable cause for law enforcement access to real-time cell phone location information; however, this decision is particularly important because it extends the probable cause requirement to stored location information. The Electronic Frontier Foundation, joined by CDT, ACLU and the ACLU of Pennsylvania, had argued for the warrant requirement that the court adopted in an amicus curiae brief filed in July. September 11, 2008"
You can view the Federal Court Decision here.