JenSense reports that we got ourselves Malicious software that targets Google AdSense ads. She explains;
A new trojan horse discovered by an Indian publisher replaces Google AdSense ads with their own ads, advertising sites including dating, sex, viagra and weight loss. This trojan is very recent, because it not only converts regular AdSense ad units, but also the Google AdSense and Firefox referrer buttons into text links.
JenSense continues by saying that there was no reports of this affecting YPN ads. In addition, Techshout has pictures showing the very similar visual appears of these ads when compared to Google AdSense ads.
Forum discussion at DigitalPoint Forums.