Hi Stephen,
If I tell it to use an NFS source (eg nfs://server/mnt/cdrom) then itin Centos5 tries to do a CIFS mount instead (I can see it running mount.cifs and in the xterm I ran virt-manager from it then asks for a password). Wow.
The correct syntax is:
nfs:nfs-server:/mnt/cdrom
I think I found it in the anaconda documentation, or in the FC6 Install guide.
Bye!