The current stable samba tree does not contain ldap support. The HEAD and TNG branches should, but are still under heavy development. When stable version are released I will document the implementation of samba in it here. Until then, you might want to take a look a document written by Ignacio Coupeau, where he describes the setup of Ldap for both of these branches.
Anyway, at this point, the smbpasswd file still has to be used. User account information is already retrieved from ldap, though. (As this is done by nsswitch, not samba.) When samba supports ldap, it should be possible to store the information that is now contained in the smbpasswd and optionally smbusers files in ldap. Whether it is possible for shares to be dynamically defined in ldap, I don't know, but I suppose it is not.