Yahoo said they will be testing Bing powered search results this month.
Since then, I haven't really seen anyone say they see it. How will you know if you are in a test group that is seeing the Bing powered Yahoo search results? Simple. The organic results should match Bing exactly.
Now, a senior member at WebmasterWorld said he saw the exact same results on Yahoo that he sees on Bing. He then eventually cleared his cookies and the results no longer matched. I trust him, he was 100% in the test group seeing Bing powered Yahoo results.
Textex, the person who saw it said:
I was seeing it for all queries. I then cleared my cookies and it was gone.
I personally tried it on several browsers and operating systems and did not see it myself. But who knows how likely I am to see these results.
Forum discussion at WebmasterWorld.