Sometimes the disk keeps showing up as "busy". It's a very annoying problem with Linux sometimes that even when you're not using a disk, it refuses to unmount.
One suggestion is to use the lazy unmount and then eject the CDROM. I won't recommend this, but it might work:
Code:
umount -l /mnt/cdrom
Note that I am not aware of the implications of a "lazy" unmount, but it works when I want to remove a CDROM from a drive when all else fails.