Currently, it seems as if Google AdWord's contextual relevancy matching systems are offline. All (most) AdSense publishers are displaying PSAs or also known as Public Service Ads. These PSAs show when Google can not find a relevant ad based on the context of your page either because their is no ad for it or Google does not yet know what your page is about. It seems like Google AdWords is having a temporary problem and it forgot what all pages are about.
Anyway, forum coverage at WebmasterWorld.