"The results of an International Advertising Association (IAA) survey find that members looked poorly on Google's proposed partnership with Yahoo last fall.
...."

http://arstechnica.com/news.ars/post/20090121-microsoft-funded-survey-drives-ad-competition-point-home.html

Here's an opposing article, which suggests that the "fear" amongst advertisers and regulators was whipped up by Microsoft and its lobbyists:

http://www.wired.com/techbiz/it/magazine/17-02/ff_killgoogle