Walter Pincus, a 71-year-old Washington Post reporter who has earned the respect and envy of his colleagues for the government contacts he has cultivated in more than 30 years at the paper, walked into a conference room at a Washington law firm two weeks ago and read a statement. "As someone who covers national security and intelligence, I depend on confidential sources more than most reporters," he told Patrick J. Fitzgerald, the special prosecutor appointed to investigate the disclosure to journalists of the identity of a covert C.I.A. agent, Valerie Plame. "My sources take a chance when they trust me...