Google has taken to the Inside AdSense blog to make the company's stance on ads that pop up over the content you want to see or pop up under your browser window very clear and very official, which is to say that they are banned. Google condemned such ads in no uncertain terms, saying that they do not "provide a good user experience", and are "not suitable" for use in Google's ad ecosystem. The policy not only disallows the use of AdSense to power pop-up and pop-under ads, but also bans the use of AdSense ads in any capacity on pages that show pop-under ads. Pop-ups are shown somewhat similar contempt, though rather than being banned outright, sites with popups have to adhere to specific rules.
This blog post is actually a clarification on an earlier rule change that instated these changes. AdSense ads cannot be shown at all on pages with pop-unders, and pages with pop-ups have to have no more than three. These three allowed pop-ups must not interfere with a user's ability to navigate the site and view content, and must not try to start any downloads that the user didn't ask for, or be in any way linked to malware. On a similar note, displaying your own ads on a frame that's showing another site's content is also not allowed, though the page that the frame originated from can show AdSense ads.
Google's hard stance against advertisements that damage the user experience is nothing new. The company has been actively working toward fighting malicious ads for some time, and has always had policies against its own AdSense ads being used in a manner that hurts the user experience. The policies surrounding the use of AdSense ads have always changed to adapt to trends in the ways that web pages are shown and ads are used. Google even went as far as to create an ad blocker for Chrome that does away with certain types of advertisements that are extremely detrimental to the user experience or are outright malicious, though that ad blocker has not actually been integrated with Chrome just yet. Besides the crackdown on pop-ups and pop-unders, AdSense's rules remain largely the same, so those who weren't using those two types of ads will have nothing to fear from the policy changes.