public static function getAllDataFromTable()
{
    $model = Mage::getModel('custom/table');
    $collection = $model->getCollection();
    $array = array();
    foreach ($collection as $item) {
        $array[] = $item->getData();
    }
    return $array;
}

/**
 * Mage_Core_Model_Resource_Iterator
 */
public function getAllDataFromTable()
{
    $ordersCollection = Mage::getModel('custom/table')->getCollection();
    Mage::getSingleton('core/resource_iterator')->walk($ordersCollection->getSelect(),
        array(array($this, 'orderCallback'))
    );
}

public function orderCallback($args)
{
  return $args['row'];
}