We're now requiring AdSense publishers to comply with the spirit of our Page Quality Guidelines. If you're an AdWords advertiser, you might already be familiar with these guidelines, which are intended to provide a better experience for users, advertisers, and publishers alike. If you use any kind of online advertising, know that these guidelines encourage publishers to, among other things, create sites with simple navigation and substantial, useful content.
This new policy requirement doesn't mean that you can't use online advertising; it simply means that if you do, you need to be sure that the way you advertise meets with the guidelines, whether it's through AdWords or through any other advertising program. However you advertise your site, it can always benefit from significant and relevant content, clear navigation, and the other points in our quality guidelines.
http://adsense.blogspot.com/2007/06/policy-updates-and-its-not-even.html