Its official: Yahoo's organic search results are now being powered by Bing.

The changes, which began yesterday, August 24th, now mean that the same search engine results will be showing up for both Yahoo and Bing. The search engine results page, or SERP for short, on each site will now be identical since the results for both are coming directly from Microsoft.
In terms of SEO, the order in which sites are ranked on each SERP typically varies based on individual search engine algorithms. However, now that Microsoft is providing the search results, and therefore the algorithm, websites will be ranked exactly the same on both Yahoo and Bing.
The companies that previously ranked high on Bing will now also rank high on Yahoo. However, companies that had high rankings on Yahoo need to be careful because they might not be ranking quite as well on Bing, based on the changed algorithm that Microsoft is providing.
These changes have been underway for about a year now; the initial contract was signed in 2009 which gave Bing the task of providing Yahoo with its backend search results while giving Yahoo the responsibility of managing the worldwide sales force duties for both companies' search advertisers.
According to Yahoo's Senior VP of Search, Shashi Seth, "This is a great milestone for Bing and Yahoo! and our customers, and we are happy to report the transition has gone smoothly and we feel great about the progress our search alliance has been making over the summer."