Alan,
I've got a sandbox set up and finally have the centos-ds server
installed and I seem to be able to add users at least.  I've been
looking at these docs :

http://www.redhat.com/docs/manuals/dir-server/

The install guide got me that far.   Then my next logical step was
"OK, how do I use this now to authenticate LInux users?"

So I just went through the Admin Guide - but nada.   Nothing.  Negatory.

    Take a look at documentation here:
    http://directory.fedoraproject.org/wiki/Documentation

    To authenticate linux users, you will have to configure you client hosts to ldap server by configuring /etc/ldap.conf
    Which can be done using GUI/cmdline via authconfig-tui/authconfig --help


Looking at the table of contents for the deployment guide, it does not
look to be of any help either.

    Look at Administration Guide
    http://www.redhat.com/docs/manuals/dir-server/8.1/admin/index.html


So what do I have to read to learn how to set up my Linux boxes to
authenticate from here?
    Read Above

Do they need local accounts too?
    Local accounts are needed for root and other service accounts

Local disks?
    Not sure what this means
What about website?  Wikis?  All currently using htpasswd.   How do I
convert those?
    For websites, you can refer to Apache authentication via LDAP

thanks,
-Alan

--
“Don't eat anything you've ever seen advertised on TV”
         - Michael Pollan, author of "In Defense of Food"