Wow, you are missing a few steps here;
-join your machine to the domain
Try the following guide;
particulary the section titled Samba ADS domain membership.
NOTE: Some distros (centos) run a version of samba that can cause you greif doing this. I found fc5 happy to work with this configuration.
After the linux box is joined to your domain
do a groupmap
that should achieve what you want,
further editing of /etc/pam.d can let you do a bit more with smb authentication