1. Put a unique, unguessable token in the URL: http://del.icio.us/rss/for/hublicious?private=847464fedca74945d7ea9a00ecd
2. Use cookie authentication: http://groups.google.com/group/npg-scintilla/feed/atom_v1_0_msgs.xml
3. Use HTTP authentication: http://daringfireball.net/2005/02/feed_authentication