I have been reading up on Horde recently and noticed that Turba can utilize LDAP. Turba is Horde's 'address book' component, http://www.horde.org/turba/ , and since we use Turba as part of SpamCop's webmail system I could not help but dream...
It would be a really cool enhancement if SpamCop used LDAP as the backend for our addressbooks. This could allow users to access their addresses via a client that supports LDAP. In this regard IMAP and LDAP would be a pretty good pairing for keeping data the same whether one uses a mail client or webmail for access. Although LDAP has been traditionally used for 'global address book' or 'shared addressbook' situations, it is possible (from what I have read) to implement individual address books including authentication to access them.
Just a thought in case it is feasible given money, time, etc.