When mounting a remote CD via NFS, and the CD is not actually mounted on the server, the mount succeeds with the server's root filesystem showing up via 'df' on the client. The remote filesystem did not appear via 'ls', but it is worrying that the mount succeeded at all (confirmed at in the server logs). The exports file contained only the cd directory. Both systems were running RedHat6, one stock, one with a manually compiled 2.2.9 kernel. The problem was also observed when the roles of the systems were swapped. If you can't reproduce it, I can provide logs, config files, terminal dumps etc.
/mnt/cdrom (unless you specified differently during the install) is located on the root file system. It is an regular directory like any other. If a client mounts the /mnt/cdrom directory over NFS it will work fine regardless if anything is actually there. When the client does a 'df' on their system, the servers root file system stats will show up because the /mnt/cdrom directory is on the root file system. This is not a bug therefore.