One of the reasons many are angered about things like leaks of details about the NSA's Terrorist Surveillance Program, is the fact that such leaks could warn al Qaeda of these efforts. That seems to have happened with the news of a manual that references a number of NSA projects - including ECHELON.
In the past, other leaks that could have aided terrorists have occurred as well. One of these was the interrogation log for Mohammed al Khatani, who was believed to have been slated to be the 20th hijacker. By publishing the log, Time Magazine gave al Qaeda an idea of what might be done. Other leaks have exposed the existence of CIA programs to keep senior al Qaeda personnel on ice to avoid giving the organization an idea of what the United States might know and on efforts to monitor financial transactions. These programs have helped break up attacks, including the plot to blow up airliners over the Atlantic this past August.
Surveillance of communications is often used as a means of gathering. How this is done is not talked about often. This is because the information that is acquired is often very good. Surveillance of enemy communications was one of the biggest factors in the American victory at Midway in 1942, and many other operations since. In 1943 Ten months later, a similar surveillance led to the mission in which Tom Lanphier shot down the plane carrying Isoroku Yamamoto. Communications intelligence also played a major part in defeating the U-boats in the Atlantic as well.
A lot of the methods used to gather intelligence in World War II (particularly codebreaking) were kept classified for decades after the Japanese surrender ended the war in 1945. Similarly, the Venona project stayed secret in the 1950s, even when information from that project could have defused controversies over the prosecution of the Rosenbergs or Alger Hiss.
In the war on terror, this is equally important. Not just for breaking up attacks, but also for mapping out the networks through a process known as traffic analysis. This can be done with just phone records (the leak of the NSA's request to various phone companies managed to tell terrorist which companies cooperated, and who company didn't). This is vital in terms of building an organizational chart of any enemy capabilities, but only if the enemy doesn't think that others are listening.
The al Qaeda manual that was reported on, though, indicates that al Qaeda is aware that intelligence agencies are listening. As a result, locating new cells will be much more difficult, making a future attack more likely to succeed. - Harold C. Hutchison (email@example.com)