I believe that the umount command exists on all versions of Mac OS X. However, it will not unmount a mounted CDROM volume but, rather, returns a "Operation not permitted" error.

A better solution is the one offered by the AM below, use the drutil command. This command should unmount the mounted CDROM volume and eject the CDROM. It doesn't matter if a Finder window is currently displaying the contents of the CDROM volumes. Read the drutil man page for dealing with multiple CDROM drives.