To add on to Iñaki's comments, make sure you do some tests from your moodle box to your LDAP server to make sure its working.
Depending on which OS you are using, I would perform ldapsearches from the moodle server:
ldapsearch -x -h ldap.example.com -b "dc=example,dc=com" "uid=test"
Or make sure that the ports on your ldap server are open from the moodle server:
Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2011-10-05 11:35 EDT
Interesting ports on ldap.example.com (xxx.xxx.xxx):
Not shown: 1673 closed ports
PORT STATE SERVICE
22/tcp open ssh
111/tcp open rpcbind
389/tcp open ldap
636/tcp open ldapssl
Just some other ideas