>line is drawn with things like online RSS readers

For starters, I'd look at the Supreme Court's opinion in Grokster. Here it is in HN-suitable pseudocode. :)

  IF $distributing_product_used_by_others_to_infringe == FALSE THEN
    FIND_NO_LIABILITY ( "None found" )
  ELSEIF $distributing_with_object_of_promoting_infringement == TRUE THEN
    FIND_LIABILITY ( "You're inducing infringement ")
  ELSE
    IF $actual_knowledge_of_infringements == TRUE and $failed_to_block == TRUE THEN
      FIND_LIABILITY ( "You're a contributory infringer ")
    ELSE
      IF $capable_of_substantial_noninfringing_uses == TRUE THEN
        FIND_NO_LIABILITY ( "None found" )
      ELSE
        FIND_LIABILITY ( "You're a contributory infringer ")
      END
    END
  END

How this applies to online RSS readers is left as an exercise for the reader...

UPDATE: A copyright lawyer friend points out via email that I neglected to include vicarious liability. I'll leave the above pseudocode as-is and note vicarious liability here as a bugfix/todo.