There is a WebmasterWorld thread named Yahoo Ads W/No 'Ads By.'? which asks how can someone remove the "Ads By Yahoo!" part of the Yahoo! Publisher Network ads?
The thread continues to explain that in some cases, it may appear that the "Ads by Yahoo! is missing but in reality, it is just hard to see. Some what like hidden text on a page.
One member says;
The background is a light yellow and the Yahoo tagline is nearly invisible because it displays in white. The Y color is their choice not mine.
Forum discussion at WebmasterWorld.