Hi,
Presumable, you can just stop the nfs services and that will allow you to unmount :
# /etc/rc.d/init.d/nfslock stop
# /etc/rc.d/init.d/nfs stop
Also, remember not to anywhere be in the /mnt/cdrom directory yourself when you unmount otherwise it will never do it...
# cd /mnt
# umount /dev/cdrom
Hope this helps