I regularly visit many forums to see the problems of different webmasters. While browsing through forums i found many postings related to Google Adsense and non-google adds and Yahoo Publishers adds on same page. I found many webmasters are confused in showing Google and Yahoo Ads on same page. From last three years i am also involved in Google Adsense and Google Adwords campaign and i constantly monitor effects of both adds on same page of my website. I placed Google ads and yahoo ads on same page of my website. Before placing contextual ads i checked my website ranking in google and it was on around 50 for most competitve keywords. I made one file that listed all keywords on which we are good position. Then after i placed both adds on the home page of my website and found that day by day our position lacking in the eye of Google.
Finally from this research i come to know that if someone put google adsense and yahoo publishers ads or any kind of other contextual adds on same page, website result start vanish. Then i wrote to Google about publishing non google contextual adds on same page and i got following reply from Google Adsense team
if you choose to place non-Google ads on the same site orpage as Google ads, it should always be clear to the user that the ads areserved by different advertising networks and that the non-Google ads haveno association with Google.
For more information regarding google adsense issue visit http://www.google.com/adsense_help where you can post your questions.
So, it is clear that you cant use Google and Yahoo ads on same page.
Once i removed yahoo adds from my webpages my ranking improved and now my websites are on good position in Google.
SEO & Web Promotion Expert.