In general, you could use Samba as a domain controller but of course it doesn't have the features of Server 2003 (and Active Directory in particular) like group policies. A better solution, since you are using Opensuse in the clients, is Zenworks
You could also check out: Likewise software
, Howto setup SUSE 10.3 as Samba PDC
, Howto LDAP server
, Howto LDAP userAuth
I am interested in the opposite problem. Having a linux-based domain controller for Windows clients.