20 million page views per month or 5 million unique visitors per month per account. You may have multiple websites. However its entirely upto google to approve or disapprove. 20 million PV doesnt gurantee a premium account. Hope that helps.
If your a premium publisher your allowed to modify the Adsense code to make it blend in more. For example getting rid of the "Ads by Google" bit Corrent me if im wrong
I think you can do pretty much you want. I also think you get a person who optimizes you ads for you.
There are lots of premium publisher.... check out ezinearticles.com (go to any article and check the skyscraper ads.. it has 10 ads on it and also notice the fonts size) wisegeek.com (go to any article and see the fonts size of their ads.) They get an option to select the fonts and the font size There are many other who fall into the category. Ani
http://www.about.com http://www.download.com http://www.propeller.com (you can see premiun adsense in stories) http://www.nytimes.com http://www.digg.com (also had premium adsense, maybe they removed it) http://www.technorati.com http://www.techcrunch.com etc