John Reese. He netted more than $500K with Adsense from Feb 05 - Mar 06 by launching 200-300 small content sites. Most of us don't have the vast resources John has but it is definitely possible. You can review the video which promotes his private marketing group here (no affiliate URL...just some staggering Adsense numbers):
http://files1.reesereport.com/vrevideo.html