Just so that I’m clear, I don’t oppose the use of anonymous sources when that is the only way to get vital information to the public. Here’s a primer on good and bad uses of anonymous sources.

Good Use of Anonymous Source (“Iraqis Stunned by the Violence of a Bombing,” New York Times):

Several days earlier, the truck, which belonged to Iraq’s Oil Ministry, had been hijacked by armed insurgents and the driver kidnapped en route from Baghdad to Falluja, according to an official at the Interior Ministry, who requested anonymity for fear of administrative punishment or reprisals from the insurgents.
“The only explanation the Interior Ministry has now is that the whole operation was arranged, and an insurgent was waiting in Musayyib to blow himself up at the location,” the official said in a telephone interview.

Bad Use of Anonymous Source (“Top Aides Reportedly Set Sights on Wilson,” Los Angeles Times):

A source directly familiar with information provided to prosecutors said Rove’s interest was so strong that it prompted questions in the White House. When asked at one point why he was pursuing the diplomat so aggressively, Rove reportedly responded: “He’s a Democrat.” Rove then cited Wilson’s campaign donations, which leaned toward Democrats, the person familiar with the case said.

In the first case, the source’s actual life would be in danger if he or she went on the record. Also, the information is important to the public’s understanding of the story.

In the second case, the source just wants to take a cheap shot at a political opponent and avoid accountability. Also, the information couldn’t be less important to the public’s understanding of this story.

I would say about 99 percent of the information attributed to anonymous sources in the American press falls into the second category, and certainly the Valerie Plame leak does. A national shield law for reporters would just encourage this kind of thing. It’s a bad idea.