Bad idea to change the ratio. I have already encoded all my videos (yes there are a lot of them) to the aspect ratio of the iphone which also involved clipping the sides of quite a bit of videos specifically so that I won't have black borders at the top and bottom. This change would now show those videos with black borders on the sides.
I sure hope they increase the screen in both dimensions to keep the aspect ratio. This would also make apps not have to deal with aspect ratio changes.
Because Apple totally cares how you've encoded all of your videos.