Assuming that it's not mechanically jammed, then rebooting while pressing the eject key (I just keep pecking at it; holding it down would probably work just as well) will activate the eject function. It should cough up the disc in that case.
If it is jammed in there (or if your drive has somehow gotten misaligned with the slot), you'll have to open up the 'book. It's not too bad (much easier than working on a clamshell or icebook, that's for certain). If you find yourself in that situation, post back. Someone here will walk you through the procedure.