The American Civil Liberties Union has conducted a study in which it requested information from over 380 state and local law agencies about policies for tracking cell phones. The results, culled from over 200 responses, show that the vast majority of departments utilize cell phone tracking information on a regular basis — sometimes routinely. The rules for accessing such information, however, are inconsistent across the country and perhaps as a result the majority of respondents do not secure a warrant before tracking. Most departments get the information directly from the carriers, though some go so far as to get their own cell tracking equipment.
The inconsistent standards are likely to continue, as the New York Times adds that the federal courts have issued conflicting rulings on what is and isn't allowed. Although the Supreme Court has weighed in on certain types of GPS tracking, a definitive judgment that applies to cell phones hasn't happened yet. In the meantime, you can visit the ACLU's site for the full documentation the organization has received from local law enforcement to see what kinds of standards are being applied in your area.