once I got a dvd stuck in my mbp.. I had to take the computer apart down to the superdrive and take that apart to get the disc out manually
edit: also.. make sure that the opening where the disc goes isn't bent closed even just a little bit.. that is what happened with mine on a seperate occasion, the "slot" was kinda warped so I had to use pliers and bend it back to the original width so the disk would come out (it was warped so little you could barely notice it, but it was enough to make the disc not come out). The disc would partially eject and go back in like you are saying. But carefully bending the warped slot back to the originally shape fixed it.. not sure if that's your problem.