WebmasterWorld Admin, trillianjedi, posted a thread in the supporters forum (paid subscription required) named Referrer based delivery for contextual advertising. That is right, he has a theory that Yahoo! searchers are more likely to click on a contextual ad that says "Ads by Yahoo!" as compared to clicking on an ad that says "Ads by Goooooogle." Same applies to a Google searcher, where a Google searcher may be more likely to click on an ad that says "Ads by Goooooogle" as compared to clicking on an ad that says "Ads by Yahoo!"
That being said, and if this theory is right, would you cloak the ads based on referrer? You currently have to deploy some sort of cloaking for ads when on the Yahoo! Publisher Network. Specifically, blocking international users from viewing YPN ads, so you won't get your site banned for quality reasons.
I suspect the results of the theory may be posted in the WebmasterWorld Supporters forum at a later date.