We collect statistics on Google AdSense advertisers, in support of our anti-web-spam efforts. The data comes from our AdRater plug-in. Unlike most metrics, ours are about advertisers, not users. Our plug-in is English-only, so there's a strong English-language bias to our data.
We just ran our report for the last 60 days. Advertiser quality is about the same as it's been for years. For 41% of the advertised domains (the one the ad links to, not the one it appears on), we can identify the business behind the ad. 36% of domains don't look too legit. Those numbers have been holding steady for three years. This is an indication that Google still doesn't vet their advertisers very strictly.
There's considerable churn in sites advertised via AdSense. Perhaps a third of the domains we've ever seen advertised are still live and still advertised. We see about 11,000 domains currently linked to by AdSense ads. This is a sample, but we've probably logged almost all the domains which get significant AdSense traffic.
Details here, if anyone wants the numbers: (http://sitetruth.net/?p=88