http://publisher.yahoo.com/rss_guide/faq.php

There's an interesting use of the XHTML namespace recommended by Yahoo
in its Publisher's Guide to RSS:

How do I prevent my feeds from being listed in the search results for
the Yahoo! Feed Database?

The "Add Content" feature in My Yahoo! allows users to search the
Yahoo! Feed Database for available feeds. To prevent your feed from
being searched in the Yahoo! Feed Database, while still allowing users
to subscribe to your content, add the following code to your feed:

<meta xmlns="http://www.w3.org/1999/xhtml"
name="robots" content="noindex" />

Any thoughts on this approach, and whether it should be included in
the RSS Profile or extended with other meta tags?