What Has Changed?
Yahoo! now has full control over mobile ads using the Yahoo! Gemini platform. Mobile under the new deal includes Smartphones & Tablets. But Now Yahoo! can run 49% of their own desktop ads (most likely with the Yahoo! Gemini platform). Yahoo! can also have Google serve a portion or all of the 49% of desktop ads.
The Bottom Line
You will need to sign up for a Yahoo! Gemini account if you want to show 100% of the time on Yahoo! For now Bing remains the provider of ads for Yahoo! Desktop search but that is going to change soon.
What about Premium Buyers (Major Advertisers)?
Yahoo! used to service both Yahoo & Bing ads. Now Yahoo! services Yahoo! ads and Bing services Bing display ads. Instead of one Yahoo! rep for all Yahoo! & Bing ads, you will now have two points of contact for advertising on Yahoo! & Bing.