http://www.youtube.com/watch?v=Zf2I9JXd9OQ&feature=g-upl

The real reason is simple ...

A graphic designer (probably) believes that the yellow line makes the Baby-Blue label area "pop" regardless of what images are used by the publisher as box art.

:) Jokes aside, Blue and yellow are very complementary colors (purple and yellow are factually complementary colors).

And yellow better divides the banner from the box art than the Cube's grey did imho. It also better fits both core and casual themes in the box arts and as such is more generic, as the WiiU should be (deeper and wider).

I'm just wondering on how are they going to determine the colors. Are the developers going to pick their favorite or will they have a special meaning?

Maybe yellow is for the better selling games or something.

im not gonna buy a wii u because of the yellow line...

Knowing the Nintendo haters I'm sure some of them will give that excuse...

its called sarcasm....

no yellow is part of the box art so it will be for every game.

He noticed. He was just saying some would say it, and wouldn't be sarcastic about it.

 This would have made a lot more sense if I knew what the yellow line was before I watched the video