Yahoo! announced that "This week marks an important milestone, as we have begun to shift ad serving for Yahoo! Search Marketing over to the Microsoft Advertising adCenter platform. This means that some of your traffic from Yahoo! is now coming through adCenter account, rather than YSM account. The ad serving transition is expected to be completed by the end of October."
"With the search transition in the home stretch, we want to make sure that you're well prepared to reach searchers on Yahoo! Search, Bing and our partner sites. We'll also try to clarify some points about the transition process," said Yahoo!.
More Info: Ad Serving Now Shifting to adCenter