A moderator of one of the forums posted a recent experience with the new "Mediapartners" bot that crawls content in order to serve targeted AdSense ads. After sending the spider through a few loops, the site's home page stopped appearing in the main search index. Coincidence? GG replies by reiterating that the two bots aren't friends and don't exchange data, and that he will ask around about the matter.
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Urg. I wish every third message didn't mention conspiracy theories around here lately. ;) Lisa, we gave the bots different names so that you could use robots.txt on one but not the other. The AdSense bots and Googlebot have never met, and don't exchange information, so they're completely independent. I'll ask around about this, but to show content ads we normally have to have or fetch the content of the page.