I seem to have this very odd issue with CentOS 6 WRT NIS. I have taken the package selection that I used with CentOS 5 and basically plopped it into my C6 kickstart file (see below). On C5 this works just fine and I'm able to log in with NIS credentials just fine. However, it looks like on C6 if you use a package selection like this, you also need to specify the yp-tools package as part of the kickstart *even though* you specify an authentication method of NIS in the kickstart. Seems like a bug to me?!?
auth --useshadow --enablemd5 --enablenis --nisdomain test --nisserver yp.example.com
%packages @core augeas autofs dstat nfs-utils ntp openssh openssh-clients openssh-server portreserve puppet redhat-lsb rsh-server sendmail sendmail-cf sudo sysstat tcp_wrappers wget xinetd yum-utils %end