the mouse trick didn't work for me, this did:
i had this problem tonight, tried everything on apple/support, no work!!!
called support, the first thing he suggested worked, apparently a secret i haven't seen anywhere else!!! i searched for two freakin' hours, and 5 minutes on the phone worked!!!
shut computer down, while holding the eject button on the wireless keyboard, turn computer on, eject as designed!!!
i tried at least 5 other suggestions at least twice each to no avail before this.
2008 intel based imac
tried;
dragging icon to trash can. NO GO
ejecting from itunes. NO GO
eject via disc util. NO GO (gave message "unable to unmount", with no clue what to do!)
reboot while holding Opt-Comm-O-F. NO GO
eject in terminal. NO GO