Of the 131,400 subpoenas and 49,700 warrants it received in 2011, only 965 were rejected. (For reference: AT&T had 103,200,000 customers that year. On average, that makes more than one subpoena for every thousand AT&T customers (although it's just possible that one very naughty customer got all the subpoenas.))AT&T indicated that it has a staff of 100 people dedicated full-time to dealing with such requests. Other carriers indicated that while federal law indicates that should reimburse carriers for the cost of collecting the requested user data, law enforcement agencies rarely do.
While most requests require a warrant or subpoena, there is also a class of requests that can be classed as emergency (for example, getting address for a customer calling 911 but unable to provide their location). The study also revealed another class of requests that worry privacy advocates - cell tower dumps. Instead of focusing on a particular user or phone number, a tower dump requests information from all users whose calls passes through a particular tower over some period of time. Tower dumps can include hundreds or even thousands of users, most of whom would not be the target of investigations, and whose private information would still end up in the hands of authorities.
Look for this to continue to be an issue.
Sources - In First U.S. Accounting of Wireless Phone Surveillance, Carriers Reveal 1.3 Million Requests for User Data, PopSci
More Demand on Cell Carriers in Surveillance, NY Times
Congressman Markey's office has posted the Carrier responses (reports).