There are reports at DigitalPoint Forums of some publishers noticing adult oriented ads being delivered via Google AdSense on sites. I personally have never seen an ad with adult content on it within AdSense (yes, AdWords but not AdSense). One example ad read;
Hottest quality babes doing the nasty
There are two reasons why these types of ads should not be displayed on Google's AdSense ads.
(1) There should be no content that would contextual match such type of adult ads. It is against the program policies for a site's content to contain any "pornography, adult, or mature content." So without the content, the ad should not match.
(2) Also, I believe it is against Google's own policy to display adult oriented ads in AdSense. This is in order to protect the publisher's readers, audience and reputation.
Forum discussion at DigitalPoint Forums.