Results from a Congressional study on the topic are being reported on the front page of the New York Times this morning.
The article reports 1.3 million requests have been made this year from a variety of law enforcement agencies, many fulfilled without judicial revue , without subpoena. Information includes content from cell phone text messages and cell tower dumps. A cell tower dump allows law enforcement to review individuals in a specific area during a specific time. AT&T claims 230 of 700 daily requests are fulfilled without subpoena. Sprint reports 1,500 such requests are made daily. The number of such requests are increasing at a rate of 12 to 17 percent annually.
Full Article: http://www.nytimes.com/2012/07/09/us/cell-carriers-see-uptick-in-requests-to-aid-surveillance.html?pagewanted=1&hp