It's true. Yahoo has owned up to "testing" an integration with Google search results. Previous to sharp eyes uncovering this unholy alliance, no official announcements were made by Google or Yahoo regarding the inclusion of Google results in the Yahoo Search engine.
Previously Yahoo was drawing its search engine results exclusively from the Microsoft Bing platform. When Yahoo re-inked their contract with Microsoft in April of this year, they added a clause allowing the inclusion of competitive results. Enter the Google.
Yahoo has gone on the record with this somewhat cryptic comment: "... We strive to create the absolute best experience for Yahoo visitors and occasionally we may run limited experiments with a variety of vendors including search providers. We have nothing further to add at this time ... "
Some say the devil is in the details, and boy is it with this deal. Yahoo has agreed that 51% of its desktop search results shall be branded with Bing sponsored ads. More importantly, Yahoo is free to choose alternative providers, whether that by Google Adsense, inhouse Gemini or others. This opens the door to a previously closed market for Google, hence the big deal.
Remember kids, this is only a test. It remains to be seen if Yahoo forms more permanent relations with other partners or is this to become a sole Google venture. This seems to reinforce the idea most web developers have that Search Engine Optimization (SEO) needs to be crafted with Google in mind.