authentications on stock CentOS failing w/o PAM

All,

Install of Virtualmin (Pro) on a stock CentOS 5.x system is failing for me. Root can not authenticate to webmin.

Looking at webmin's error log, it's only complaint is that Authen::PAM is missing. Yes, I know this complaint has been mentioned before and the standard response is that it's not needed unless NIS or LDAP is being used.

However:

Installing Perl-Authen-PAM (from virtualmin's repo) fixes the problem; after restarting webmin, root can now authenticate.

This is a pretty serious issue for me...

And no, the system is only using passwd/shadow for authentication.