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If true at all, I think there is nothing in the Adsense T&C that could prohibit this. Unless this page could be considered a "thank you" page.
Anyone else thoughts on this?
(I thought that web adverts were still paid CPM until I signed up with AdSense.)
Many still are. If anything, there's been a trend back toward CPM; Google has been offering site-targeted CPM ads on the AdSense network for a while now, and at least one major research firm is predicting that display ads (which tend to be CPM) will outstrip search ads (including AdSense text ads) in revenue growth over the next few years.
Getting back to the OP's question, I see nothing in the AdSense TOS that would forbid asking for donations on pages with AdSense ads. Whether the OP should ask for donations is another story. Does the presence of an electronic "tip jar" on a site convey an image of professionalism and credibility that inspires trust in the site and its ads?
Very few people would think of 'donating' by clicking ads since almost no one in the public even knows clicks can generate revenue. The small fraction who know about internet advertising assume ads are running based on impressions, or prepaid ads, not ppc.
In fact, most people who I have talked to in person assumed the ads were running free, either as a product information service to visitors, or to merely fill-in empty space on the webpage. When it comes to marketing and advertising at least 95% of the public have basically no clue about it.