The new resource feeder plugin works a treat:

  def index
    feed_options = {
      :feed => {
        :title       => "Comments on '#{@tag}'",
        :description => "Comments made by Web Connections users on the tag '#{@tag}'",
        :link        => tag_url(@tag),
        :language    => 'en-au'
      },
      :item => {
        :title => :feed_title,
        :description => :body_html,
        :link => lambda { |r| tag_url(:id => @tag, :anchor => "comment-#{r.id}") }
      }
    }
    respond_to do |format|
      format.rss  { render_rss_feed_for @tag.messages, feed_options }
      format.atom { render_atom_feed_for @tag.messages, feed_options }
    end
  end