You started with and excellent analysis Paul and you end up with anti Google rant
So do lots of people use Ad blocker this days? I know of pop up blocker, but I am not sure adsense blocker is common place yet.
The reason I think AdSens may not be working is the same reason banner ads went out of fashion, it eventually become "part of the furniture", you tune you mind to ignore it an concentration your surfing experience so you don't even notice the Ads.
It could also bee the amount of unique visitors you get, forum tends to attract the same members again and again so less people see the ads in a forum compared with a site that attract lots of unique visitors.
Temi