Assuming that the LDAPS server does not have security holes, exposing it to the wide Internet should be no more risky (and no less) than exposing a HTTPS Web server. With LDAPS (SSL outside, traditionally on port 636, LDAP protocol in it), the authentication requested by the server will be performed under the protection of SSL, so that's fine (provided that authentication passwords are strong, as usual).
... with one caveat nonetheless. A big part of HTTPS security is that the browser, i.e. the client, makes sure that the server's certificate is correct: verification of all signatures with regards to a set of trust anchors; revocation status checks; verification that the intended server name really appears where it should in the server's certificate. If you access the LDAPS server through some software, then that software should apply the same kind of verifications; but I doubt most LDAPS clients are that thorough.