Are they logging in locally or via SSH?

Locally.  Remote logins via ssh work just fine as the home directory is auto-mounted and ssh can find its keys.

I think I solved the problem, but am out of the office today to fully test it.  It involved setting the default realm and adding some encryption types to the /etc/krb5.conf file.  What I still don't understand is what has changed in CentOS 6 that causes a kickstarted system not to be able to authenticate users whereas a CentOS 5 system can.  I need to do a few more installs to track down the root cause, and then I'll post an update here.

Alfred