Requires that you've set up a ~/com.apple.loginitems.plist file defining what apps you want to launch. It would be nice to automate converting from using the system's copy of this file to this custom-located one.

launch_OSX_Login_Items.rb

#!/usr/bin/env ruby

require 'rexml/document'

def set_up_Growl
  if `ps -ax | grep GrowlHelperApp\.app | grep -v grep` == '' then
    system( 'open /Library/PreferencePanes/Growl.prefPane/Contents/Resources/GrowlHelperApp.app' )
  end
end

set_up_Growl

loginitems_plist_xml = `plutil -convert xml1 -o - -- ~/com.apple.loginitems.plist`

REXML::Document.new( loginitems_plist_xml ).each_element( '/plist/dict/dict/array/dict/dict/dict' ) { |login_item_dict|
  index_of_Path_key = i = 0 ; elements = []
  login_item_dict.each_element { |e|
    elements << e

    if e.text == 'Path' then
      index_of_Path_key = i
    end

    i += 1
  }

  login_item = elements[index_of_Path_key + 1].text
  system( "open '#{login_item}'" )
  sleep( 1 )
}