Extension Attribute for Display Serials

Cool, your example works on my system (MacBook Air with Thunderbolt Display)
If your interested here a two other ways to get the same value that I was inspired to share while looking at your post

#!/bin/bash
serial=`system_profiler SPDisplaysDataType | awk '/Display Serial Number/{print $NF;exit}'`
echo "<result>$serial</serial>"

and a little more complicated example that I have been working on for generically parsing system profiler content as an extension attribute (still work in progress , as each script needs to be modified for the given key ). Worth nothing in both examples that they take the first Display Found

#!/usr/bin/python
__author__ = 'Zack Smith @acidprime'
__version__ = '1.0'

import plistlib
import subprocess


system_profiler = '/usr/sbin/system_profiler'
spx_key = 'spdisplays_display-serial-number'

# Working on this to make it more generic

def genReport():
  spx = {} 
  # This is our key value schema
  SPDisplaysDataType = {
    spx_key : spx_key,
  }
  _dataTypes = {
   'SPDisplaysDataType': SPDisplaysDataType,
  }
  dataTypes = _dataTypes.keys()
  # run the system_profiler command with data types
  arguments = [system_profiler,"-xml"] + dataTypes
  getspx = subprocess.Popen(arguments, stdout=subprocess.PIPE)
  spxOut, err = getspx.communicate()
  rootObject = plistlib.readPlistFromString(spxOut)

  # Parse datatype top level keys below
  for array in rootObject:
    for _dataType in _dataTypes:
      if array['_dataType'] == _dataType:
        _dataTypeSchema = _dataTypes[_dataType]
        for key in _dataTypeSchema:
          for item in array['_items']:
            for spdisplays_ndrv in item['spdisplays_ndrvs']:
              if spx_key in spdisplays_ndrv.keys():
                  spx[spx_key] = spdisplays_ndrv[spx_key]
                  break

  return spx
spx = genReport()
try:
  print '<result>%s</result>' % spx[spx_key]
except KeyError:
  print '<result></result>'

https://gist.github.com/2142589

This while much bigger does have one nice feature, if you change the key value it will pull the first value for that key , i.e.

spx_key = 'spdisplays_resolution'

would display something like

<result>1366 x 768</result>

Hey Zack your bash one has a typo

echo "<result>$serial</serial>"

Should be echo "<result>$serial</result>"

Works good tho!!