Integrating WordPress with LDAP shouldn't be difficult. Now it isn't. Simple LDAP Login provides all of the features, none of the hassles.
Having a single login for every service is a must in large organizations. This plugin allows you to integrate WordPress with LDAP quickly and easily. Like, really really easy.
If you use this plugin, and find that it serves your organization's needs, please consider a donation. I have spent many unpaid hours supporting and updating this plugin. A plugin like this would easily retail for $200 or more. I offer it for free, with only the request that you consider giving back if you are financially capable of doing so.
If you need support, I recommend you leave a comment on the appropriate post on my blog: http://clifgriffin.com/2009/05/13/simple-ldap-login-13-for-wordpress/
If you need a customization or change specific to your install, I am available for hire. Shoot me an e-mail: clifgriffin[at]gmail.com
Simple LDAP Login adds an authentication filter to WordPress that authentication requests must pass. In doing so, it makes several decisions.
This is simply a high level overview. The actual logic the plugin employs is more complex, but hopefully this gives you an idea, philosophically, about how the plugin accomplishes what it does. If the plugin is unable to authenticate the user, it passes it down the chain to WordPress. (Unless security mode is set to high, which will disable this functionality.)