My PPC system tracks ROI based on exact keyphrases. Google's system allows this easily with their [exact match] functionality. Yahoo's Standard Match is similar, but adding new keyphrases constantly trips the "new keyphrase is too similar to a keyphrase already in the list" message. Because of this I don't track the ROI of individual keyphrases in Yahoo at all, I only track ad group ROI there.
Has anyone found a way to track individual keyphrases effectively in Yahoo?