Nicolas,
I disagree about states. You say either extensions are registered or not. Unfortunately if FOP2 can't connect to the AMI then they are seen as registered whether they are or not. Anyway it's your product so the final decision is up to you. I agree that if FOP2 isn't connecting to the AMI because of a configuration issue then its not your problem.
I'm still confused as to why line state changes are not displayed with Asterisk 1.4 in my installation. With 1.6 there's no problem. However I have noticed that displayconnects is marked as no when you type manager show user admin on the asterisk console despite the fact that I have configured it to yes. Can this explain the fact that the line state isn't displayed?