My opinion is that it is true in a sense.... partly based on what I heard in Jennifer Slegg's interview with the director of online sales and operations for Google AdSense, Kim Malone (listen to the interview in the first show with host Jennifer Slegg on WebmasterRadio - ClickThis Show).
It's been a few weeks since I listened to it, but if memory serves, Kim responds to Jennifer's question about how traffic from lower quality sources affects AdSense. I'll have to relisten to get the details right I think, though I believe she made comments about avoiding bad traffic sources that would bring in less qualified/targetted traffic, and that traffic like that could have a detrimental effect on your results with AdSense (e.g. due to bad ROI for the advertisers could cause smart pricing, which would then lower your CPC). I don't totally understand smart pricing (does anyone? haha) but I believe this would cause "all" of your clicks to be devalued, not just those from the bad traffic sources that were the cause.
Google's certainly aware of the search terms or link text that led a visitor to the page as well, so I would assume they probably factor that into their results (e.g. if you had a camera review page, a visitor who clicked a link to "digital camera reviews" is more qualified than one who clicked a link to the same page that said for instance "free camera stuff" or something). That side of it I'm guessing on though, so I'll have to look for evidence 
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