AdSense API beta is a service that enables you to integrate Google Adsense in your site. By using AdSense API, you can enable users using AdSense functions without leaving your site. Here is what they said:
Using the AdSense API, you can enable users to perform a variety of AdSense functions without leaving your website, including the following:
– Create an AdSense account
– Manage an AdSense account
– Create and modify AdSense for content ad units and link units, AdSense for search boxes, and Referrals
– View detailed reports to monitor performance and earnings
With AdSense API you can get more earnings from publishers that you signed up. And you will get $100 more if the publishers get $100 less than 180 days after they registered. But anyway, not all site will be accepted cause your site must have minimum 100 000 page views per day.
More Info: Introducing the AdSense API beta