People are already using the Kindle app on the Nook Color - the rooted ones, of course. But honestly: an eBook reader app on an eBook reader just to read the competition's eBooks, you consider that
to be a "killer app"?
And actually that picture above is a developer unit of the Nook Color, in the lab at B&N when they put Angry Birds on it almost 2 months ago. The root was just released about 48 hours ago; that Angry Birds demo was to show people what the Nook Color is quite capable of.
Now with the root in place and improving each day, so will the Nook Color.