A bunch of Yahoos?
The Yahoo Publisher Network is sort of a text based advertisment similar to Google ads. If you click to apply to join, it points out that your web site should be US focused.
According to this post in BoingBoing, 'Publisher Policy. Item '11.l' stated that I will not "display all or part of the Ad Unit to any user located outside the US". In other words, I can't allow users outside of the United States to view my pages if there is a Yahoo ad on the page!'
The only offered solution to prevent your site being in violation of this term, is to only allow worldwide web visitors from the US; to block the site off from the rest of the world.
I know that it gets shortened to "the web" but shouldn't the "worldwide" bit mean something?