A WebmasterWorld thread asks if it is allowed to place AdSense ads on pages that require you to skip the ad to see the content.
There are many sites that have what are called interstitial ads. These ads act as a gateway to the content you want to see. Typically, you click from a site to see an article of interest. But before the content is shown to you, you are taken to an intermediate page, with an ad, that says this article is provided by Company X. After about ten or fifteen seconds, the page will automatically redirect to the article, or you can manually skip the ad.
To place Google AdSense ads on those pages is not allowed. I received confirmation from Google on this.
Forum discussion at WebmasterWorld.