Why exactly should users not be allowed to mount a CD? I can understand limiting hard disks, but not removable storage.
Not sure why it doesn't work for you...maybe you're missing something else? Here's a piece of my /etc/fstab which allows users to mount, and execute files on, cd/dvd:
/dev/cdrom /media/cdrom auto unhide,ro,noauto,user,exec 0 0
/dev/dvd /media/dvd auto unhide,ro,noauto,user,exec 0 0
man fstab should give you more info