Arrgh I knew I shouldn’t have put it in, but got a mini-CD with a copy of Photo Rescue (a $29 value!) which I need, so I just stuck it into the MacBook and it is stuck. http://forums.macrumors.com/showthread.php?t=406613 says never do this. But how was I to know?
The solution is ridiculous. Turn off your MacBook and then give it a good shake the mini CD comes out. 🙂