Sticking a butter knife (or anything else) in the drive should be a last resort if your computer is out of warranty. It might work, but it's also a great way to completely bork your drive.FullmetalZ26 gave some good suggestions. Also, you might want to try shaking the computer a bit to see if you can jostle the disc free. Be gentle, and make sure the computer's off when you do it.
I would recommend visiting an Apple Store if you have one nearby and asking a Genius what they would recommend. They might tell you to just bring it in and they'll take care of it. They may or may not charge you for it, but even if they do, it might be worth it.