What DWallach said. The sampling microphones have to be right next to the playback speakers (the buds in your ears) for it to work.

It might sorta work if you strapped the macbook to your head.

If the noise is a single point-source, it's theoretically possible to install a limited version of noise cancelling on the source itself. An example would be on something that has a noisy engine, you could install a system of incredibly loud speakers around that engine. I seem to remember reading about someone doing that on a diesel train as an experiment.

But for any sampling point *between* the source and your ears, you simply can't have a catch-all noise canceler algorithm. Any noise heard by that sampler will be different than the noise heard by your ears, and would thus add to the ambient noise rather than subtracting from it.