I found rolodap since I last posted, and installed it for a tryout. It's quite good, but comparing that to horde, it occured to me that you can actually do the same in horde's address book (turba). That would be better integrated than rolodap, which would be bringing yet another application into the picture!
The main thing with both of them is, if you want to use LDAP, you need to think about how you're going to set up the LDAP configuration, and do it on e-smith so it doesn't clash with the LDAP entries automatically maintained by the user management.
I've a pretty good idea how to go about it, but I've not had time to concentrate on it properly.
I think I'd still be limited to maintaining the address book via horde's web interface, but it would be accessible by any LDAP enabled client. I use Outlook (and sometimes OE) for most of my email, and they always treat LDAP as read-only. I'd consider switching Windows client if I knew of one that will write to LDAP!