I just had this problem and, of course, jumped to the internet to try to find a solution. And to my surprise I found very little information.
I got a little bit lucky and was able to get out of this one, so I just went through the registration process to post my experience.
Apparently, there is a small pin that blocks the superdrive entrance to avoid inserting 2 discs at once, or I guess to prevent a disc from coming out. By some strange reason, the pin was blocking the entrance, but it didn't recognize any disc inside (eject button wouldn't do a thing and rebooting wouldn't work either). So, I tried to make the macbook think there was a CD inside.
I turned on the computer and waited for it to start up normally. Then, I closed the lid, and it went to suspend mode. I used a credit card, and carefully inserted it into the superdrive and looked for a mechanism that could be pushed, and inserted a good part of the credit card. I tried to do this with the card touching the top to avoid scratching the cd lens.
Once that was done, opened the lid. The computer then detected something inside and did the noise to eject a drive. Slowly, I removed the Credit Card, looking for the pin at the same time... it was gone. =)
I tried a DVD (Zeitgeist if you are curious) and it works =)
Disclaimer: Not an expert and this can be potentially dangerous to your superdrive / lens / credit card... this worked for me but that's it. Do it on your own risk! =)