In ruby 1.8 you don't have option to serialize hashes sorted. You can find many monkey patches on the internet which allows this. Unfortunatelly there are numerous errors in them. I would like to keep here most bug-free version.

sorted_hash.rb

class Hash # :nodoc:
  undef_method :to_yaml

  # Replacing the to_yaml function so it'll serialize hashes sorted (by their keys)
  #
  # Original function is in /usr/lib/ruby/1.8/yaml/rubytypes.rb
  def to_yaml( opts = {} )
    YAML::quick_emit( object_id, opts ) do |out|
      out.map( taguri, to_yaml_style ) do |map|
        keys.sort_by do |k|
          k.to_s
        end.each do |k|
          map.add( k.to_s, fetch(k) )
        end
      end
    end
  end
end

Revision #2 fixes problem with:

#<TypeError: wrong argument type Fixnum (expected String)>