Since signing up with a click fraud program things have gotten rather interesting.I also have ads from Overture.
Just to make sure, is your click fraud company telling you adwords hits are coming from search.yahoo.com or is it telling you that you're getting hits from yahoo? If it's just hits, then it's the Overture ones.
[pagead2.googlesyndication.com...]
url=http://www.example.com/widgets&....&ref=http://search.yahoo.com/search?p=widgets&...
What this tells is that the user clicked on the AdSense link on example.com, but he was referred to example.com from yahoo.com's search page.
So the user did not click directly from yahoo's page to your website but through another website. These websites typically tend to be "Made for AdSense" sites that use SEO techniques to rank high on Yahoo. But one advantage, atleast for me, is that if there are more such sites, I needn't go through the pain of advertising on Overture at all :)