ruby script/plugin install http://dev.rubyonrails.org/svn/rails/plugins/atom_feed_helper/

# from PostsController
def index
  @posts = Post.find(:all, :limit => 25)
    
  respond_to do |format|
    format.html
    format.atom
  end
end

# from posts/index.atom.builder

atom_feed(:url => formatted_people_url(:atom)) do |feed|
  feed.title("Address book")
  feed.updated(@people.first ? @people.first.created_at : Time.now.utc)
  
  for post in @posts
    feed.entry(post) do |entry|
      entry.title(post.title)
      entry.content(post.body, :type => 'html')

      entry.author do |author|
        author.name(post.creator.name)
        author.email(post.creator.email_address)
      end
    end
  end
end

... will produce Atom feed.

There is currently no advanced documentation for this plugin.