From Inside Adsense:

Category filtering – a new feature we’ve been developing to give publishers more control over their ads. Google Adsense extending beta to publishers in a number of English-speaking countries including, but not limited to, Australia, Canada, India, Ireland, the U.K., and the U.S. Category filtering lets you prevent ads from up to 5 specific categories such as religion, politics, & dating from appearing on your pages. Your filters will be applied to ads in English, regardless of how they’re targeted. In addition, you’ll be able to see the percentage (%) that each category contributes to your earnings, which can help you understand any revenue impact you might notice as a result of filtering.

http://adsense.blogspot.com/2009/08/extending-category-filtering-beta-for.html

To see whether category filtering has been enabled for your location, sign in and visit the Ad Review Center, located under the AdSense Setup tab!