This is something I'd wish I'd blogged about because I wasted time remembering how to do this.
This assumes an Ubuntu 8.10 install
First you must sync up the users:
php -c /etc/php5/apache2/php.ini /var/www/moodle/auth/db/auth_db_sync_users.php
Then your must sync up the enrollments
php -c /etc/php5/apache2/php.ini /var/www/moodle/enrol/database/enrol_database_sync.php
Will flesh this out later...