As announced but as expected and seen already, Bing is, on occasion, powering Yahoo results - so they can test Bing powered Yahoo results in the live results.
You have a pretty good chance of seeing Bing powered Yahoo results. As I mentioned at Search Engine Land, up to a 25% chance of seeing Bing organic results on Yahoo. And for the first time, I personally see them matching. Here is a screen shot, side by side, showing them matching exactly, even the images:
The red portion is the only portion of the "organic" section that is not the same. Why, well those are not really "organic" results, they are news injected results.
The Bing Search Blog also posted this morning, emphasizing:
For webmasters, it’s important to be familiar with how the Bing crawler interacts with your site. After the full algorithmic transition is complete, you only need to optimize for one crawler (Bing), as we will provide Yahoo! with results from our index.
Forum discussion continued at WebmasterWorld.
