It just came to my realization that Google's Adsense recently changed their program policy. You can now put Adsense on the same page along with any other contextual advertising system. You must however, differentiate Adsense from the other advertising system, such as using a different color and background. I can now test using Adsense and other contextual advertising programs on the same page, compare stats side by side and in a few months, I'll be able to see which has the consistently higher cost per click (CPC).
The excerpt from Google's guidelines is below. read it on Google's adsense help site
You're welcome to display Google ads on the same site or page as other third party advertisements provided that the formatting or colors of the third party ads is different enough from that of the Google ads. In other words, if you choose to place non-Google ads on the same site or page as Google ads, it should always be clear to the user that the ads are served by different advertising networks and that the non-Google ads have no association with Google. If the formats are naturally similar, we ask that you choose different color schemes for the competing ads.
Our intent with this policy is to be as fair to our advertisers as possible and to maintain the integrity of the AdWords and AdSense programs. You can learn more about this, and all our policies, on our program policies page.
I guess this was done with relatively low fanfare because I haven't seen it much. Just on one thread in Digitalpoint.
UPDATE: Adsense can be used with other contextual advertising programs provided that the other advertiser also allows this. I checked YPN's TOS and as of this time it seems they do not allow it. Its better to send an email and ask YPN before you implement them both on the same page. An excerpt from YPN rules and conditions below
Exclusivity. For any webpage or RSS feed that includes the Ad Code, you agree not to display or link to any other advertising (including but not limited to any listing) that is mapped to or responds to the content of the Ad Page.