[CentOS] CentOS 6 PXE boot:Unable to download the kickstart file

We have CentOS 6 manual installation working by PXE booting from a RHEL5.6
PXE/TFTP server.  However, when we add a Kickstart file in the PXE
configuration:

    kernel CentOS-6-i386/vmlinuz
    append load_ramdisk=1 initrd=CentOS-6-i386/initrd.img network
ks=nfs:130.226.86.4:/u/rpm/kickstart/ks-centos-6-clean-i386.cfg

then the CentOS 6 client install reports "Unable to download the kickstart file".
The console 3 reports "failed to mount nfs source".
We believe the NFS server is OK because we can install CentOS5 clients in
this way.

Perhaps this problem is related to this RHEL6 bug?
https://bugzilla.redhat.com/show_bug.cgi?id=653655

However, on the centos list I see some people who apparently go Kickstart
to work with CentOS 6 - but how did they do it?

Question: What special configuration is required for either 1) PXE or
2) the DHCP server?

Thanks,
Ole

-- 
Ole Holm Nielsen
Department of Physics, Technical University of Denmark