I am looking for howtos or instructions on setting up Samba to use an AD server for authentication instead of using a Samba password file. Pointers to documentation helpful, especially the right document to look at on http://samba.org/samba/docs/ .
It sounds like you want Samba to join to a Windows Server running Active Directory as a Domain Member You will need to set-up Winbind as well using either pam_smb or pam_winbind. I can't remember exactly. Lastly, you will need the winbind nss module activated in /etc/nsswitch.conf for passwd/groups.
Here is a good guide for ArchLinux. It worked quite well for me and may be feasible for other distributions, too.