Search Engine Journal, Search Engine Roundtable and Webmasterworld are discussing Google AdSense hints. The basic idea is that this is a feature not available to most publishers to "Override contextual targetting by telling google what keywords a page relates to". According to posters at Webmasterworld, your site has to be of a certain traffic, and complaining about poor keyword targeting can help.

I'm not able to say much about this, but just a few points: If someone tells you they know of the code for keyword targeting in AdSense, do not use it. Your account will be suspended. This, among several other premium AdSense features, are only available to those Google gives permission to. Since every AdSense page load goes right back to Google's servers, they will know the second you implement it, and they will shut you down, either temporarily or permanently.