Since Bing is now powering Yahoo search in the US and Canada I guess this move was inevitable:
Last week, Bing began powering the organic search results for English-language Yahoo! Search in the United States and Canada. This week, advertisers can begin moving their Yahoo! Search campaigns into Microsoft Advertising adCenter. We expect the paid search transition to be complete by the end of October, which will allow your adCenter ads to reach more than 159 million searchers in the U.S. and 15 million in Canada1 on Yahoo! Search and Bing.