Update using Zend_Feed_Rss

addPingdomProbesToDebugList.php

<?php

require_once 'app/Mage.php';
Mage::app();

$xml = new Zend_Feed_Rss('https://my.pingdom.com/probes/feed');

$debugIps = Mage::getStoreConfig('dev/restrict/allow_ips');

if($debugIps) {
        $string = $debugIps.",";
} else {
        $string = null;
}
foreach($xml->channel->item as $item) {
        $parts = explode(";",$item->description);
        $ip = trim(substr($parts[0],4));
        $string .= $ip .  ",";
}
$config = Mage::getModel('core/config');
$config->saveConfig('dev/restrict/allow_ips',$string);
echo "Debug IP list updated to ".$string."\n";