An old WebmasterWorld thread was recently refreshed with the question, when is it coming?
One member recently received a survey from Yahoo's Lori Love Vice President of Customer Solutions. The survey mentioned the editor, but had no hint as to when of if it will ever come. One advertiser was told by his platinum specialist that Yahoo advertisers may see something in the first quarter of 2009. He did add that the editor will be lacking many features that you currently find in AdWords Editor.
Yahoo has been under a lot of stress recently, primarily over their search ads division. I wonder if the editor is a priority right now for them.
Forum discussion at WebmasterWorld.