In tomorrow's build, any LDAP directories that you've set up for autocomplete will now show up in the addressbook.
When you select them, the results pane will be blank, so quick search is the only thing you can do with them. Soon we'll be landing additional UI to add / modify the LDAP directories from the addressbook, and to make them usable from the addressbook sidebar and the select addresses dialog. There are several know issues, so query for bugs with "nab-ldap" in the status whiteboard before logging bugs. -Seth
