From Ian Ayres posting over at Freakonomics, a collection of reasons given for source anonymity by various news publications following a 2004 change to NYT’s confidentiality policy.

A few of the cited reasons:

• (Spoke on condition of anonymity because) …of the delicacy of the negotiations.
• …he did not want to be seen as speaking for the president.
• …he was not authorized to divulge results.
• …many people do not know she smokes.

Ian’s writeup touches on some of the variations in both phrasing and the nature of the justification in various citations.  He also mentions that “…by 2005 there were 9,451 articles using the phrase.”  Which is the sort of thing that makes me drool a little.