When using Magento EE, and especially when flat category is active, requesting the "url_key" attribute for a category fails because it is not populated in catalog_category_flat_store_*. This method fixes that.

enterprise_category_url_key.php

<?php
  /**
   * Get category URL key for Enterprise
   *
   * @param Mage_Catalog_Model_Category $category
   * @return string
   */
  public function getEnterpriseUrlKey(Mage_Catalog_Model_Category $category)
  {
      $resource = Mage::getSingleton('core/resource');
      $conn = $resource->getConnection('core_read');
      $tableName = $resource->getTableName(array('catalog/category', 'url_key'));
      $select = $conn->select()
          ->from(
              array('url_key_table' => $tableName),
              array('url_key' => 'url_key_table.value', 'store_id' => 'MAX(url_key_table.store_id)')
          )
          ->where('url_key_table.entity_id = ?', $category->getId())
          ->where('url_key_table.store_id IN(?)', array(Mage_Core_Model_App::ADMIN_STORE_ID, $category->getStoreId()))
      ;

      $row = $conn->fetchRow($select);

      return $row['url_key'];
  }
?>