AdSense's new Allowed Sites feature is a step in the right direction.

In the past, it has been too easy for the unscrupulous to get their competitors or enemies banned from AdSense through such means as repeated clicking on ads. While I expect this problem to continue for some time, Google has introduced a new feature which will help AdSense publishers protect themselves. Publishers can now at their option specify which domains or subdomains they wish to allow ads to appear on; although the ads may be displayed on sites which are not on the allowed sites list, no impressions or clicks will be counted for those ads. This prevents two possible situations that could lead potentially to trouble: competitors or enemies will no longer be able to steal someone's AdSense code and plaster AdSense ads on sites which violate the AdSense terms of service, and people who mistakenly put in an incorrect code when placing ads on a site that violates the terms of service will also no longer get innocent people into trouble. Truthfully, neither situation happens all that often...but both do happen sometimes so it is good that Google has addressed this issue. Hopefully, getting incorrectly banned from AdSense will be the least of the worries facing web publishers in the future.

Is using the Allowed Sites feature a good idea for everyone? I've begun using it myself, but there is one issue all publishers should be aware of before they create their own Allowed Sites list. Archival sites which cache old versions of pages, such as archive.org and various search engines, will no longer generate revenue for you from those cached pages unless you also add those sites to the Allowed Sites list. You'll have to do that manually at the moment. I would hazard a guess that most people make next to nothing from impressions on cached versions of their sites, but those that do might be better off if they avoided the allowed sites feature for now.