I really loved my N9 screen as well and I did not know AMOLED was vulnerable to burn ins. I don't remember of any symptoms on my N9 but I haven't really checked. However navigation/GPS applications may be the real issue as NX500 pointed out: they stay up with the screen on for literally hours continuously, I don't think this is the case for the battery icon in Harmattan unless the lockscreen is disabled (I don't remember actually, is it visible on the lockscreen, isn't it a different/bigger icon at a different position?).

I like to enable "keep display on when charging" on my phones so that I don't have to unlock them all the time when plugged to my computer or when ssh-ing, usually it does not last hours but I might forget to manually lock the screen overnight sometimes, and this would be a significant concern with a screen notoriously vulnerable to burn ins. But well, the N9 screen looked so good that I can't really say I would not like an AMOLED here again.