After installing Zimbra 3.1.4 on OpenSUSE 10.1 I tried using the Administration panel to change authentication settings. The settings used were
Authentication Mechanism: External LDAP
ldap://ldap.test.com Port 389
ldap filter: uid=%u
LDAP search base: ou=people,dc=test,dc=com
Use Bind DN/Password: Checked
Bind DN: uid=moodleldap,ou=systemaccounts,dc=test,dc=com
bind password: test

I then enter a username and password from a user in the people OU and the test is successfull.

I then check the Authentication tab in the Administration Panel and magically the LDAP bind DN template: field is blank and Use DN/Password to bind to external server: is set to No

Does Zimbra just forget to save these settings?

When trying to log into zimbra mail, users in the ldap database fail to authenticate because Zimbra doesn't use the appropriate Bind user and password.