An April 15th, a retired NSA (National Security Administration) engineer (Thomas Drake) was indicted for providing classified information to a reporter. Drake had been at it for several years.
These leaks, mostly in the Washington, DC, area, are quite common, with about a hundred a year since the 1990s. Most (60 percent) are not even investigated. And since 1995, only two of these 600 investigations have led to a prosecution. The other prosecution was of Lewis Libby, who was convicted not of leaking the identity of a CIA employee (another government employee, who was not prosecuted, was responsible), but of lying to the FBI during the investigation.
Many leaks are not investigated because they were deliberate, sometimes even authorized by a senior political official. Many of those investigated go nowhere because you need strong evidence to nail the leaker. Thomas Drake was caught using encrypted email to do his leaking. The NSA is very good at deciphering encrypted material, but they won't talk about what they might have done to break this case. Another problem is that if the leaker has powerful allies within the government, the investigation can easily be derailed, or stonewalled.