I think the corrections do happen, after the complaint volume gets loud enough.
A couple of weeks ago, AP had a story out of Albany, GA about the reaction to the Bush-Guard-CBS story which made no mention of the forged documents. I called the Albany, GA bureau and left a message for the reporter (he apparently holds down the fort there), and also called the Atlanta bureau. Others certainly did the same in addition to e-mailing. PowerLine Blog came out hard with criticism. A sentence saying that "questions have been raised about the authenticity of the documents supporting the story" was added within a few hours of the story's initial release.
So results happen, but I suspect it takes a LOT of griping to make it happen.