I'm able to retrieve the drenched in user title using NSUserName(), but how do you obtain the Open Directory "directory domain title" or any other domain he is owned by? i.e. like the notion in Home windows in which a drenched-in user is DOMAINUSERNAME, where DOMAIN is his local machine, workgroup or Active Directory domain.

You might have the ability to do that with Directory Services. Alternatively, browse the new Open Directory APIs introduced in 10.6 you've got a choice between a Core-Foundation-based API and a Cacao-based API.