My 9 key sometimes does not respond on my new Macbook. When this happens, it returns after I strike the key hard a few times. It then comes back completely. The first time this happened, I thought some small particle had lodged under the key and it was now gone. However, the same thing happened again today. Of course I've restarted it a number of times.
Most likely a defective keyswitch. Your best bet is to call AppleCare or schedule an appointment with a Genius at your local Apple Store. If it's a hardware problem, not much else you can do... Good luck.