I wouldn't call it a legal obligation yet. I think what we've seen is a belief that a Security Council authorization is politically and even morally desirable for NATO. Although it is true that under article 51 of the UN Charter force cannot be used except in self-defence or as part of a collective security operation, that doesn't mean that NATO always has to act under a council authorization. However, for these wars of choice like Libya, I think it has become the practice. That may build up over time into customary law, but I wouldn't yet call it a rule of law.
On October 16th, 2012. See this statement in context.