eGroupware may be the answer...
http://www.egroupware.org/

You can host your own server or pay them to host one for you.
http://www.stylite.de/EGroupWareHosting

Full LDAP access, shared addressbooks, private addressbooks, lists, decent web interface. vcard, and DAV access.

Also does calendaring...

I set it up at home on a virtual machine and sync all our various linux boxen to it to various degrees.

Thunderbird/Lightning link to the calendar via CalDAV
Thunderbird uses LDAP for the addressbook.
When TB can edit LDAP then it'll be cool.