I did some research for a client with regard to pretty much this question on Snow Leopard. What I came up with was a solution that involved using PHP to sychronise address book data between FileMaker and an LDAP directory - see php.net for the references to LDAP. Individual Address Books were then configured to have access to the LDAP directory.
thanks Chris. I will pass this along
Just try our new Plugin: fm4CardDAV on our website : http://fm4all.net . FM4CardDAV is a Filemaker Plug-In to manage your Contacts in Filemaker and synchronize this with your CardDAV Server.