default-value-for-new-field.php

<?php
$type = 'product_kit';
$field = 'field_availability';
$value = 'in stock';

$result = db_query("SELECT nid, vid FROM {node} WHERE type = :type", array(':type' => $type))->fetchAllAssoc('nid');
foreach ($result as $nid => $values) {
  $fields = array(
    'entity_type' => 'node',
    'bundle' => $type,
    'deleted' => 0,
    'entity_id' => $values->nid,
    'revision_id' => $values->vid,
    'language' => LANGUAGE_NONE,
    'delta' => 0,
    "{$field}_value" => $value,
  );
  db_insert("field_data_{$field}")
    ->fields($fields)
    ->execute();
  db_insert("field_revision_{$field}")
    ->fields($fields)
    ->execute();
}