cannot log in: 6.0 upgrade broke PAM / pwdb

[Ben Liblit <liblit cs berkeley edu>]

I recently upgraded a Pentium box from mixed redhat-5.2 / Rawhide /
Starbuck to redhat-6.0.  I now find that I cannot log into the system,
either via "login" or via [xgk]dm.

If I boot in single-user mode, I can gain root access and peer at the
syslog.  It appears that the upgrade broke PAM or libpwdb:

    PAM_pwdb[###]: get passwd; pwdb: structure is no longer valid
    login[###]: FAILED LOGIN 1 FROM (null) FOR root, Authentication
                failure

I have re-downloaded and reinstalled the util-linux-2.9o-13,
pam-0.66-18, pwdb-0.58-3, and glibc-2.1.1-6 RPM's, but to no avail. 
I've even tried rebuilding pwdb from its SRPM.  No good.

The system uses traditional Unix /etc/passwd and /etc/group, no shadow
files or NIS.  Nobody can log in:  root, non-root, empty password,
non-empty-password....

Help!  :-(