I know next to nothing about LDAP structure but I'm certain it's possible for us to authenticate against Zimbra's LDAP server. It's already there and running so this would help centralize our authentication methods. The only thing I'm interested in authenticating with currently is SVN which is just apache authentication via mod_auth_ldap.
As I've said, I know very little about LDAP and nothing about how zimbra lays it out. It shouldn't be very difficult for someone who knows what they are doing..
http://httpd.apache.org/docs/2.0/mod/mod_auth_ldap.html