A thread at Search Engine Watch forums details how to use NO SCRIPT tags to display an alternative ad to those visitors (such as an affiliate ad or an image related to the site or a partner site) so you can attempt to monetize those visitors who will not see your AdSense or YPN ads.
A simple appending of NOSCRIPT to your Google AdSense code solves this problem
and displays ads when all Google Ads are disabled. Note that this is not modifying the AdSense script whatsoever and only runs when AdSense is disabled so there isn’t any terms and conditions issues.
Just make your HTML look like this:
… Google AdSense Code….
…. some affiliate code here….
incrediBILL, who posted the tip, is correct that it is completely within the terms and policies to do this.