Yep, use services.module. It provides APIs for Drupal.
Or:
<?php
chdir('/path/to/drupal/');
require_once('includes/bootstrap.inc');
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
edit: Per the comments, for command-line usage, you have to spoof $_SERVER stuff that Drupal relies on (particularly the domain name):
<?php
// set some server variables so Drupal doesn't freak out
$_SERVER['SCRIPT_NAME'] = '/script.php';
$_SERVER['SCRIPT_FILENAME'] = '/script.php';
$_SERVER['HTTP_HOST'] = 'example.com';
$_SERVER['REMOTE_ADDR'] = '127.0.0.1';
$_SERVER['REQUEST_METHOD'] = 'POST';
// act as the first user
global $user;
$user->uid = 1;
// change to the Drupal directory
chdir('/path/to/drupal');
// Drupal bootstrap throws some errors when run via command line
// so we tone down error reporting temporarily
error_reporting(E_ERROR | E_PARSE);
// run the initial Drupal bootstrap process
require_once('includes/bootstrap.inc');
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
// restore error reporting to its normal setting
error_reporting(E_ALL);