I've tried to have ldap authentication login on the ldap directory using login/password of the user to identify. I've set something like that in my conf/local.php :
$conf['auth']['ldap']['usertree'] = 'userLogin=%{user},ou=people,dc=xxx,dc=xxx';
However, ldap auth module was still doing an anonymous bind and search (which didn't return any result on my setup, because of access rights).
In fact, the problem seems located in inc/auth/ldap.class.php. On line 67 (in 20060309 version), code tries to access the 'usertree' option through "$cnf['usertree']" instead of "$this->cnf['usertree']" (see attached one-line patch). With this correction, authentication works as expected. (this problem seems to be present in darcs sources too, but not tested).