<?php

function _webform_component_mapping($node) {
  $mapping = array();
  $components = $node->webform['components'];
  foreach ($components as $i => $component) {
    $key = $component['form_key'];
    $mapping[$key] = $i;
  }
  return $mapping;
}

$mapping = _webform_component_mapping($node);
$submission->data[$mapping['newsletter_signup']][0]) == 'YES';