Washington Post reporter Walter Pincus on anonymous sources (from the Summer 2005 issue of Nieman Reports):

But no matter what legal protections exist, journalists should pause before handling information received from people who demand anonymity. Reporters should avoid promising anonymity to sources if it is being offered simply to encourage the source to say something in a dramatic or damaging way that the source would not say on the record. This use of anonymity harms the profession and diminishes the value of the confidentiality given to those who are whistleblowers