LDAP Authentication + Drupal Registrations possible issue
If anyone is interested, here are some methods of resolving this:


You can do a force creation of all ldap users before putting your
application into production. Edit I've written a module for this: http://drupal.org/sandbox/mrryanjohnston/1308882
You can require everyone to authenticate with their email addresses.
    This, too, requires a few modules and a little bit of figuring
    out, but it can work. This module is the result of some of those
    efforts: http://drupal.org/sandbox/branana/1300714