I am fairly sure this USED to work, but after updating to the more recent versions (probably after the major rewrite), it doesn't seem to work any more...
Mac OS X's OpenDirectory LDAP server supports and can verify against multiple uid values for a given dn. Now, if the wordpress user id doesn't match the first uid listed the authentication will fail.
I wonder if it is related to this from a Drupal similar problem?
Anyway, thanks for thinking about it. I may try to twiddle the source and see if I can determine anything further, but not sure I want to mess with a production site and don't know when I'll be able to set up a test one.