In a blog post here, he says, in the context of reputation:

The way quality works on the web is through links … it works because reputable writers make links to things they consider reputable sources … One’s chosen starting page, and a nurtured set of bookmarks, are the entrance points, then, to a selected subweb of information which one is generally inclined to trust and find valuable.

A great example of course is the blogging world. Blogs provide a gently evolving network of pointers of interest. As do FOAF files. I’ve always thought that FOAF could be extended to provide a trust infrastructure for (e.g.) spam filtering and OpenID-style single sign-on and its good to see things happening in that space.