元ネタは https://gist.github.com/2986414/32a6c6cb9a9cc99e96f4cd9026513d0644ba0511 ここ。
2年前のものだったのでNokogiri 1.5.0で動くように改修。Hash classを拡張。

xml2hash.rb

# USAGE: Hash.from_libxml(YOUR_XML_STRING)
# http://movesonrails.com/articles/2008/02/25/libxml-for-active-resource-2-0
require 'nokogiri'

class Hash
  class << self
    def from_libxml(xml)
      begin
        result = Nokogiri::XML(xml)
        return { result.root.name.to_s.to_sym => xml_node_to_hash(result.root)}
      rescue Exception => e
        # raise your custom exception here
      end
    end


    def xml_node_to_hash(node)
      # If we are at the root of the document, start the hash
      if node.element?
        if node.children.present?
          result_hash = {}

          node.children.each do |child|
            result = xml_node_to_hash(child)

            if child.name == "text"
              if !child.next_element.present? and !child.previous_element.present?
                return result
              end
            elsif result_hash[child.name.to_sym]
              if result_hash[child.name.to_sym].is_a?(Object::Array)
                result_hash[child.name.to_sym] << result
              else
                result_hash[child.name.to_sym] = [result_hash[child.name.to_sym]] << result
              end
            else
              result_hash[child.name.to_sym] = result
            end
          end
          return result_hash
        elsif node.attributes.present?
          # TODO
        else
          return nil
        end
      else
        return node.content.to_s
      end
    end
  end
end

attributesが入り組んでいると取れないのだけど簡単な構造ならHash化できる