First, we don't support officialy alternative authentication backends, so there is a lot to learn with an LDAP implementation. Based on your experience and feedback, we may be able to make it official.
To implement LDAP support, here are the 2 steps i see:
1) Create a new user backend in Site/Users/Ldap.php or elsewhere.
You should mock what you see in the Simple.php backend. For a first version, i believe supporting the 'getUser' method is enough. It should return a PHP array with the 'id, 'username', 'email', 'fullname' fields.
Then, to active it, in dist/site.php, add the following code:
$site->setUsersBackend( new Ld_Site_Users_Ldap() );
2) Create an LDAP Authentication adapter. Hopefully, maybe you can just use the Zend Framework one. http://framework.zend.com/manual/en/zend.auth.adapter.ldap.html
The authentication backend is not yet configurable in La Distribution, so for now you'll have to edit Ld/Auth.php and change it in the 'authenticate' method.
$adapter = new Zend_Auth_Adapter_Ldap($options, $username, $password);
$result = $auth->authenticate($adapter);