As our expert User Kappy explained to another person:
Five ways to eject a stuck CD or DVD from the optical drive
Ejecting the stuck disc can usually be done in one of the following ways:
1. Restart the computer and after the chime press and hold down the left mouse button until the disc ejects.
2. Press the Eject button on your keyboard.
3. Click on the Eject button in the menubar.
4. Press COMMAND-E.
5. If none of the above work try this: Open the Terminal application in your Utilities folder. At the prompt enter or paste the following:
If this fails then try this:
Boot the computer into Single-user Mode. At the prompt enter the same command as used above. To restart the computer enter "reboot" at the prompt without quotes.
If the suggestions so far haven't worked, here's a non-official way of removing a stuck CD: http://www.youtube.com/watch?v=n7NYW9Dr448
(at your own risk, etc. but it looks pretty straightforward - though I haven't tried it myself)