A DigitalPoint Forums thread notes that Google has changed their AdSense feedback form. In the past, clicking on an ad's "Ads by Goooogle" link would take you to a special page. Now it takes you to product promotional page that looks like this.
You can then scroll down and click on the blue link that reads "Send Google your thoughts on the ads you just saw." That will open up a DHTML form, where you can give feedback on the ad or report violations.
Here is a screen capture if you don't see it.
Forum discussion at DigitalPoint Forums.