Hi Opus,
You can use Open LDAP to connect OS systems to various directory services that support LDAP. Samba is a way to read the windows directory from linux and share files. Microsoft supports LDAP in Active Directory but you may have to to build the query or purshase interfaces. Samba does that for you but I think it's still beta or in dev?

Anyway you can use an LDAP query browser to figure out what your looking for and build that into an openLDAP thingy. So yes, using Light Directory Access Protocol (LDAP) directory services can talk to each other.

This thing kicks ass BTW.