In recent comics like Avengers and Avengers Undercover it is mentioned that A.I.M island is a nation and so the avengers can't attack them. When did this happen, is it explained in another comic, or did it just happen without explanation?
According to the Marvel Wiki this happened (or was revealed) in Fantastic Four #610 http://marvel.wikia.com/Barbuda
Barbuda was later sold to A.I.M. , where they decided to establish their own sovereign nation and become public.