Samba can only emulate an NT4 Domain which is not Active Directory.
Samba-tng can participate in Active Directory trusts, but is still not a Domain Controller.
That said the following link shows how to edit the configuration files to allow kerberos, ldap, and samba to emulate the NT4 Security Accounts Manager and allow an NT4 domain running on linux. The guide is for gentoo, so you'll have to install the packages with the redhat tools, but the config files are what is important in this guide.
http://gentoo-wiki.com/HOWTO_Impleme...ba_as_your_PDC