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I tested my theory by putting a unique phrase in an AdWords ad of mine which was appearing on these sites, and sure enough, the sites quickly went to the top of the organic results for that phrase.
I can see why it happens: the sites concerned are all big enough to be allowed to run their Adsense ads in their own styles, so the text of the ads does appear in the HTML, unlike standard Adsense presentation. But it does seem nonsensical.
[edited by: tedster at 4:44 pm (utc) on Jan. 9, 2008]