Is ext/dom enabled in your build? And loaded? :)

Yes, ext/dom is enabled ---------------------------------------------:
$ php -i | grep -i dom
dom
DOM/XML => enabled
DOM/XML API Version => 20031129


I'll modify the test script to verify it also ----------------------:
<?php
echo 'Verify DOM is loaded:' . PHP_EOL;
$r1 = new ReflectionClass('DOMElement');
var_dump($r1->getName());
echo PHP_EOL;

echo 'Does DOMUserData exist?' . PHP_EOL;
$r2 = new ReflectionClass('DOMUserData');
var_dump($r2->getName());
?>

With expected output ------------------------------------------------:
Verify DOM is loaded:
string(10) "DOMElement"

Does DOMUserData exist?
string(11) "DOMUserData"

And actual output ---------------------------------------------------:
Verify DOM is loaded:
string(10) "DOMElement"

Does DOMUserData exist?
PHP Fatal error:  Uncaught exception 'ReflectionException' with message 'Class DOMUserData does not exist' in /opt/phpversions/php-bin/php-5.3.0/test.php:8
Stack trace:
#0 /opt/phpversions/php-bin/php-5.3.0/test.php(8): ReflectionClass->__construct('DOMUserData')
#1 {main}
  thrown in /opt/phpversions/php-bin/php-5.3.0/test.php on line 8

ReflectionException: Class DOMUserData does not exist in /opt/phpversions/php-bin/php-5.3.0/test.php on line 8

Call Stack:
    0.0002     624160   1. {main}() /opt/phpversions/php-bin/php-5.3.0/test.php:0
    0.0004     625432   2. ReflectionClass->__construct() /opt/phpversions/php-bin/php-5.3.0/test.php:8


This bug report was prompted by PEAR bug #14640, where phpDocumenter encounters the same ReflectionException when attempting to document the DOMElement parent class of a user's code class that extends DOMElement.  Declaring an empty dummy DOMUserData class in the user's code solves the issue.

This bug has been fixed in SVN.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.

type is mixed not DOMUserData

Thanks Rob... any chance you could also patch it on the 5_2 branch?