Caydel reports that Google is beta testing a keyword based competitive ad filter solution for Google AdSense. Currently, AdSense publishers can block ads from showing on a URL by URL basis. It appears that Google may be testing a feature that will allow AdSense publishers to block ads from showing via a keyword list. If you look at the help documentation it shows nothing of disallowing ads by keyword and only discusses disallowing by URL. No release date information has been suggested and no official confirmation from Google has been released.