How exactly does PAM figure into things? Was: Alternatives to LDAP

[Jim C <jcllings@tsunamicomm.net>]

I've implemented an OpenLDAP server /client here on my local network 
with the eventual idea of attempting cross-platform authentication.

In order to do this, I followed the instructions in Vincent Danen's 
article on OpenLDAP.   They can be found here:
http://www.mandrakesecure.net/en/docs/ldap-auth.php

Now it is a great article (really!) but he doesn't seem quite sure if 
PAM needs to be configured for OpenLDAP authentication or not.
According to him you don't really need PAM for authentication because 
nss_ldap and pam_unix will do this.

My question is as follows:

Under what conditions will configuration of PAM be required?