Here I use it to render an .xml.builder. For Rails <= 2.1.1 the trick is to stub the controller with something with a .content_type, thereafter it is no longer necessary.

render_xml_builder('lib/data_feed.xml.builder', :products=>products)

def self.render_xml_builder(file, locals={})
  builder = ActionView::Base.new
  builder.instance_variable_set '@controller', OpenStruct.new(:content_type=>'un/important') #only needed for Rails <= 2.1.1
  builder.render( :inline => File.read(file), :type => :rxml, :locals => locals )
end

Happy rendering!