Check this great footage made in air from the cockpit of a Boeing 747. The pilots filmed an Airbus A380 passing under their Boeing 747 - just 300m away. It was filmed above the South China Sea and the pilots of the Boeing got informed by a Hong Kong air traffic controller that the A380 would come very close! That is why they took out the cam and filmed it - wow!
And for those of you who can't wait. Jump to 40sec mark ;-)