Two days ago, Google Adsense, just clarified the policy in regards to pop-ups and pop-unders. These changes in policy come in a time where Google is preparing the launch of a Adblocker for Chrome that can block these ads.
The update forbids Publishers that are using Adsense to use any type of pop-unders. Publishers will be able to use pop-ups, however not more than 3. Popups can´t interfere with site navigation, change user preferences, initiate downloads, or distribute viruses.
Google Adsense will also ban publishers that load any software that triggers pop-ups, modifies browser settings, redirects users to unwanted sites, or otherwise interferes with normal site navigation.
Popunder is a window that appears behind the browser window of a website. A popup appears over the browser content.