"In a letter to Rep. Darrell Issa, R-Calif., a Justice Department official said the privilege applies to documents that explain how the department learned that there were problems with the investigation called Operation Fast and Furious."
ADDED: I'm going to add my "Obama is like Nixon" tag. I think a lot of Americans, when they hear "executive privilege" think of Nixon. And, unfortunately for Obama, we've been hearing plenty of talk about Watergate lately, what with the 40-year anniversary of the break-in. Most notably: "Woodward and Bernstein: 40 years after Watergate, Nixon was far worse than we thought." Ironically, that was a mainstream media effort to help Obama.
But Obama has suddenly chosen to look like Nixon. It must be worth it. And without the documents, we must speculate about what is in them.
ALSO: This creates an occasion to look back and see when Presidents other than Nixon have invoked executive privilege. I'm going to rely on Wikipedia, so correct me if I'm wrong. After Nixon, the next President to invoke executive privilege was Bill Clinton, in 1998, trying to keep aides from testifying in the Lewinsky scandal. The district court judge ruled against him. Clinton also used executive privilege to negotiate the terms of his own testimony in the scandal.
George W. Bush used executive privilege a few times, as you can see at the Wikipedia link.