Just recently, Yahoo switched over their search engine to display search results powered by Microsoft's Bing. With this change, their advertisement service will also change over from Yahoo! Search Marketing to Microsoft's adCenter. The transition is scheduled to complete by the end of October, which then users will be able to manage just one advertisement account for both sites, however until then users will have to manage two separate accounts.
The decision to combine the two advertisement services, as explained by the adCenter Blog, will help advertisers to contact a much greater market than previously. Now they will be able to reach Yahoo's market and Bing's market with just one tool. They are touting the ability to get to more than 159 million searchers with a single ad. Both companies will look for feedback from advertisers as they finish the transition.
Yahoo! Search Marketing is already gearing up their customers for the big changeover. To get started and prepared for the transition they have supplied you with an "adCenter" tab which will prompt you for your adCenter information so that Yahoo can start transferring over your ads. They do note that adCenter will offer a few different features from Yahoo's, which can be viewed here. For those whom are already on adCenter, Microsoft has a tool to help you start optimizing your advertisements for both of the search pages.
The change is suspected to be finished in October for the US and Canada search markets.