Even in best case scenario, I'm not sure GPS would be accurate enough, as it's only accurate down to few centimeters, you'd probably need more if you'd like to perform that kind of acrobatics as shown in the video. And what about the height? You could of course use GPS, but apparently (http://users.erols.com/dlwilson/gpsvert.htm), average consumer devices have error of few meters. Maybe we could use laser range finders pointed at the ground, but what if the quadcopter tilts? Or what if another quadcopter flies underneath it? Of course it's not unsolvable, but it'd probably require more computing power in the quadcopter.

What do you think?