Anonymous sources are dangerous.
If they are unidentifiable, who's to say they're telling the truth? And what about when your source makes a claim, that you decide to report, and then you get sued for libel or defamation? What happens then? But, at the same time, what about Deep Throat--the anonymous source who provided information to The Washington Post in 1972 about the Watergate scandal? That was news.
I think, in context of today, that journalists need to use intuition as to whether or not to report the leaked information. Is the source telling the truth? What are his/her motives for telling the information? Is it worth the risk and the potential outcomes?
But I also think it's important to keep one's word. If you say the source will remain anonymous, make sure that he/she does. Our book The Mind of a Journalist told the story of a Republican Party activist, Dan Cohen, who sued the Minneapolis Star Tribune and St. Paul Pioneer Press & Dispatch for not keeping his identity concealed. The two reporters attempted to keep his identity confidential, but their editors felt naming him would make the story more credible. Cohen was awarded $700,000 in damages (Willis 22).
However, the anonymous source Deep Throat was highly useful. Where does one draw the line with anonymous sources?
Well, it's a thin, squiggly line that no one can really see. How's that for an answer?
I think a reporter must ask himself/herself the questions asked earlier: telling the truth? motives? worth the risk? So much depends on these answers, and, ultimately, on the reporter's and editor's gut feelings. Society is often told what is news by journalists--will this story be news, or just potential to be sued?