An intersting study on what percentage of a web blog is covered in advertising on top blogs. Serverdome actually really mesured the ads on each blog in his study. You may be a bit surprised by the results. There are screenshots and results for each blog.
Another study that could be done is the ratio between traffic, ad space and revenue.
Do you need more proof that blogs make money online?
On the other hand Connectedinternet wrote a review of TNX.net and I really can not see anyone earning much from their program by putting links all over your site and at the ridiculously low rates they charge.