This is part of the reason I selected wordpress for our internal intranet, I discovered the existance of this simple, yet powerful plugin http://wordpress.org/extend/plugins/simple-ldap-login/ that links the WordPress install into the companies active directory via LDAP, this is even more simple than setting up Openfire LDAP and for that reason I’m not going to ramble on about it, just install WordPress on your local network, click the link to the plugin page and admire the simplicity!
Big thanks to clifgriffin