Mobile phone AMOLED displays are great in many respects, as they are able to render high contrast image and saturated colors. There is just a minor flaw that's reported by more and more users of AMOLED users around the world. It does not only happen in Nokia AMOLED, but also in Samsung, LG, HTC, and many other manufacturers' devices.
The problem is only apparent if you display a completely black image in a dark room, and it is very difficult to capture the defect on a photo. The easiest way to see it is to turn on the phone's empty "open applications" screen or camera standby, and look at the phone in complete darkness. The photo above comes from a user that took it with a good quality photo camera.
If you encouter any of such defects appearing on your Nokia N9 screen, you can find some consolation visiting this thread in talk.maemo.org and sharing your problems with the others. The issue is not particularly annoying as it is only apparent in complete darkness, and only when the phone displays a dark image, so chances are you will never see it on your N9.