As of today, Google will require AdSense publishers to meet the same page quality guidelines it holds its AdWords advertisers to with their landing pages. According to the Inside AdSense blog, the guidelines are meant to "encourage publishers to, among other things, create sites with simple navigation and substantial, useful content."
The requirement has been added to the Site and Ad Behavior section of the AdSense Program Policies:
Publishers using online advertising to drive traffic to pages showing Google ads must comply with the spirit of Google's Landing Page Quality Guidelines. For instance, if you advertise for sites participating in the AdSense program, the advertising should not be deceptive to users.
Google recently began cracking down on so-called "made for AdSense" sites, and reportedly terminated several publisher accounts as of June 1.
In addition, publishers are now allowed to place up to three link units on a page, instead of the previous limit of one per page.