TheMadScientist - 7:09 pm on Nov 10, 2012 (gmt 0)
is it maybe possible that Google looks at the quality/ratio of visitors on a site that do/don't click adsense ads, plus how they respond to the ads (buying signals?) and factor that in as a "quality" rating?
Google often gives info in 'Google speak' and very precise wording that's easily misunderstood/misinterpreted, but I have not known them to flat out lie, so unless something I missed has changed, AdSense specifically is not used by the search teams ... Ads in general, their placement, their ratio to text, etc. are used, but it's the same for an AdSense publisher as it is for someone selling ads on their site of the same size, positioning, layout, etc.
To answer the opening questions specifically:
Is AdSense a negative signal for serps?
Can AdSense account be toxic?
Sure, if you get married to it and plaster them all over the place like the AdSense team would love for you to do, then it definitely can be, but if you plastered your own ads in exactly the same manner it could be toxic too.
One thing I think it's important to remember when deciding what to do WRT AdSense placement is: The search teams and ad teams at Google generally have polar opposite goals and balancing the goals of each to work together on your specific site(s) is completely up to you ... If you try to follow the exact advice of both to the letter, you could easily end up with nothing more than a headache and a site that either doesn't rank or doesn't sell anything.