Hi
in my experience the real problem in ldap shared address book is in the client; I mean: setting up a ldap tree, even in SME, is simple, but there aren't email client that can WRITE on it..
so you need a connector for outlook or thunderbird, but all the connectors I know are not free, and the free ones aren't so stable to be used in production.
another way is a ldap addressbook with a web interface..
my 2c
Stefano