The ethics of using anonymous sources are an intricate pancake indeed. Note the high ethical standard on display in this US News & World Report article, for instance:
Republican strategists trying to game Sen. Barack Obama's choice for a running mate are focusing more and more on the possibility that he might pick former House Majority Leader Dick Gephardt, a friend of labor and blue-collar workers. "Gephardt is the one we're most afraid of," said a key GOP strategist and Bush ally.
The reporter knows that this unnamed GOP source is using him to "game" the Democrats on the issue of the vice presidential pick. In other words, he knows that he is being screwed with, and plays along, even letting the GOP operative not give his name, because... well, just because! He has access, dammit!
I fail to see how there is any ethical reason whatsoever for granting anonymity to someone who is clearly interested solely in propaganda. That this is of course SOP is pretty goddamn depressing.