Howtoforge has an article on configuring a CentOS server running Samba as a member of a Windows 2003 domain. An excerpt from the HOWTO says:

The intent of this article is to show you how to configure your Linux machine and Samba server to participate in a Windows 2003 Active Directory domain as a Member Server using Kerberos authentication. This involves using the security = ADS security mode in Samba.

Check out the complele HOWTO for more details.