A WebmasterWorld thread named Buying AdSense Ready Site discusses the pros and cons of buying a site that is for sale, for the purpose of making AdSense revenues.
Performing a Google search on ready adsense sites you get the WebmasterWorld thread, but then many listings of sites that are for sale for this purpose. Let's not forget the eight or so AdWord ads on the right side of the page.
But be smart if you do buy one of these sites. Make sure they are indexed. Make sure the content is unique. You know the run down. I like this line by Hobbs;
When you pay peanuts, do not be surprised to get monkeys!
Just be smart about it, add your own flavor to these sites.
Forum discussion at WebmasterWorld.