Despite Yahoo having a deep search & ad alliance with Microsoft, it seems like Yahoo wants to add more ad providers to monetize their own properties at least. I am guessing, their partner, Microsoft, just isn't doing it for them.
Yahoo announced Google is now an "non-exclusive" display ad provider on "various Yahoo properties and certain co-branded sites" specifically using Google's AdSense for Content and Google's AdMob services.
Soon, you will see Google Ads on Yahoo properties. Is this Marissa Mayer's doing? Being a long time Googler, now CEO of Yahoo...
Anyway, expect to see Google ads on Yahoo soon.
Forum discussion at WebmasterWorld.