I’m not sure exactly how apple did their ads, but a quick a dirty way would be to make sure you’ve got a crack-ton of light on that greenscreen, and pull your actor far enough away from it so that none of the spill is hitting them. That way, they’ll be black against the greenscreen.
If they’re wearing an ipod, the you may be able to mess with the levels of the layer that your actor is on, adding some contrast to the elements that you want to pop out, after keying out the background.
It’s quick and dirty, and it will still require some experimentation to get the right look, but it can work.