Bloglines - Feed Access Control Standard for RSS and ATOM:
"We are proposing (and have implemented) an RSS and ATOM extension that allows publishers to indicate the distribution restrictions of a feed. Setting the access restriction to 'deny' will indicate the feed should not be re-distributed. In Bloglines, we'll use this to prevent the display of the feed information or posts in search results or any other public venue. If other readers and aggregators use the information in the same way, and publishers of feeds, including services that let users create feeds, implement this standard, we could make significant progress toward making feeds truly safe for non-public information. We think that's a pretty cool idea.
For technical details on the RSS and ATOM extension, refer to this document: