Sprint Nextel provided law enforcement agencies with its customers' (GPS) location information over 8 million times between September 2008 and October 2009. This massive disclosure of sensitive customer information was made possible due to the roll-out by Sprint of a new, special web portal for law enforcement officers.
The evidence documenting this surveillance program comes in the form of an audio recording of Sprint's Manager of Electronic Surveillance, who described it during a panel discussion at a wiretapping and interception industry conference, held in Washington DC in October of 2009.
It is unclear if Federal law enforcement agencies' extensive collection of geolocation data should have been disclosed to Congress pursuant to a 1999 law that requires the publication of certain surveillance statistics -- since the Department of Justice simply ignores the law, and has not provided the legally mandated reports to Congress since 2004.
I've only quoted the summary of the essay. The rest is worth reading, and presents some pretty insane things (like service providers selling customer information to the government because it's very lucrative--with many having departments open 24 hours per day, for example).