As part of the Yahoo-Microsoft Search Alliance, Yahoo! has been transitioning the backend for their organic search to the Microsoft search platform. Search results for Yahoo! are already aligned with Microsoft for the United States, Canada, Australia, Brazil, and Mexico, while expansion to the rest of the world is expected before too much longer. Apparently API access will shut down on September 15, 2011, so it's unlikely to be much longer before Site Explorer officially disappears. According to Yahoo! Site Explorer will shut down once the transition of organic results in all markets to the Microsoft Search platform is completed, which is expected later this year. source