Hello Joao,
I'm using SER and Asterisks-based system, with centralized LDAP backend. To access LDAP I use SOAP and DSML.This is now used for every provisioning/management/billing/ivr activity in the system. In future I plan to have centralized phonebook based in LDAP.
I think that having centralized LDAP directory and accessing it from clients via SOAP/XML is a best option. If security is an issue, SOAP/XML Digital Signature and Encryption could be used here.
Maybe we will live until times when hardware vendors will support SOAP clients in their phones, or at least XML browsers or some sort of thin clients. I see SNOM is doing something in XML - maybe worth checking.
I think that will be the soft-phone manufacturers that will first adapt idea of central phonebook, based on SOAP message exchanges with centralized LDAP directory servers.