My sister-in-law has an iBook that she bought on our recommendation. A few days ago she reported that there was a DVD (rented from the local Lackluster Video) that wouldn't eject. I searched high and low and the collective advice of the Internet was that it wasn't really user servicable for Sister-in-Laws, so I told her to take it to the Apple store. The genius there gave it a quick try and then said it would cost over $400 to send it in for service.
She was going to fly out here for a visit soon anyway, so when she got here, I'd just take the case apart and get it done.
But I thought about it some more. One of the pages my searching got me to was a page at Apple that mentioned that the drives had to be operated in the normal horizontal position and that other orientations were not supported. That got me thinking.
So I asked her over the phone to hold her iBook upside-down, but without covering up the slot, then press the eject button. The disc summarily popped right out.
So next time a disc get stuck in your slot-loader Mac laptop, try holding the machine at various angles to see if you can get a little help from gravity.
Note that this applies to disks that are "hardware" stuck - that you can hear the mechanism trying to spit out. If it's "software" stuck, then just reboot the machine and hold the mouse button down and it should spit it out.