Both the lawsuits were filed by the Electronic Frontier Foundation (EFF). One of them ( ) was filed in 2006 and accuses AT&T of violating privacy law by illegally collaborating with the U.S. National Security Agency (NSA) to wire-tap and mine the phone and email conversations of millions of ordinary U.S. citizens.

The other lawsuit ( ) was filed in 2008 on behalf of AT&T customers and seeks to stop the NSA's "dragnet" surveillance practices, and to have them declared by the courts as an illegal and unconstitutional activity.

The government has argued that it is within its rights to conduct such surveillance activity in the interests of national security. It has maintained that the NSA and the telecommunications companies that gave it the data it sought should be granted immunity from prosecution. A failure to give them such protection could result in the disclosure of critical state secrets, the government has contended.

The appellate court will decide whether to dismiss the cases or let them proceed.

Cindy Cohn, legal director for the EFF, said the implications of the two cases are far-reaching. "This is one of the only chances we have to bring the rule of law to government surveillance of Americans," Cohn said. "The government has argued that the whole thing is a state secret because of national security [implications]."