[pilif]

You only get that dialog for the devices you add. The public key added by the NSA or Apple themselves does not trigger the dialog.

(explanatory note as the sarcasm in the comment might not have been obvious: I do not know whether such a facility exists in the services or not, so this might or might not be true.

But: The fact that it's possible that this "feature" already exists or can easily be added in the future, potentially even without an update of the client leads me to my current opinion which is that iMessage is not secure and all traffic is open to Apple, rogue employees at Apple and whatever government Apple is cooperating with).