Locate the little tiny hole on the drive door. (The shelf-like disk holder that normally ejects) When closed, you will find on it a small hole.
Step2
Bend a paper-clip out and put it in that hole. You will feel it hit something inside, when it does, VERY gently press the paper clip in. The paper-clip will be pressing a button inside that allows the drive to open.
Step3
You have your disc back.
Tips & Warnings
This is also true on iMacs. Their disk drive hole is beside the front casing.
Remove the paper-clip after you get the disk out.
Sometimes re-booting your machine will also allow the disk to eject.
Do not jab the paper-clip into the drive door hole, as this may break it.
on 8/9/2008
another way is pressing the left mouse button while rebooting. It is not widely known since it is relatively new, it was invented because of copy protected audio cds caused thousands of macs having stuck CD.
Sadly if your mouse is not Apple branded wireless (e.g. logitech), this won't work on wireless keyboard/mouse configurations. Plugging a wired USB mouse (any brand) temporally could be practical.
Novice 
Comments
Heart-C said
on 8/10/2008 sounds easy enough to remember! 5 *
Ilgaz said
on 8/9/2008 another way is pressing the left mouse button while rebooting. It is not widely known since it is relatively new, it was invented because of copy protected audio cds caused thousands of macs having stuck CD.
Sadly if your mouse is not Apple branded wireless (e.g. logitech), this won't work on wireless keyboard/mouse configurations. Plugging a wired USB mouse (any brand) temporally could be practical.
2besure said
on 6/12/2008 Thanks for this great tip.
chromedaffodils said
on 6/12/2008 Let me know if you need more help, I will try to help. :-) Take care!