Do we really think that such a measure could pass the U.N.? Sure, there are some nations that would vote for such a resolution, either out of their own desire to ban the private ownership of firearms, or even because it's anti-U.S. But, think about the nations such a measure would affect. Germany (H.K.), Austria (Glock), Canada (Para-Ordnance), Italy (Beretta),China and Russia (all those SKS rifles we love, and the ammo). Money out of their pockets, and remember money does mean something.
Plus, as was pointed out along the way, the U.N. doesn't have the clout to pull off something that big and enforce it. Look at the situation in the Sudan. Genocide is actively occurring, and the matter is being debated in the U.N. Now, picture the same debate about the U.S. and arms control. They can't put together a peacekeeping force to stop the violence in the Sudan. Can they really muster an armed invasion of the U.S.?
And, even if it was with the U.S. Government approval (which would likely be the end of the Constitutional government we enjoy, think of the practicalities of the measure. A house-by-house search across the nation. There aren't enough troops to pull that off, even with minimal armed resistance. And, I think we'd be more than minimal armed resistance...