Slightly more relevant, since we're talking about sourcing in Washington when reporting on government, is the sourcing policy of the Washington Post. This Editor & Publisher article is the latest I found on that. In addition, here is the New York Times' policy on using confidential sources.
UPDATE: Apologies for those of you who can keep straight the ever-shortening list of media conglomerates, but Newsweek is owned by The Washington Post, so one would think that the WaPo sourcing policy applies to Newsweek. Again, if you find out, let me know.
More later. Right now I have to "work" to pay "bills."