Ok, this is just a thought, and nothing more. But you know how the touch sensitive buttons on the NS are hard to see when they are not lit up? Sure it gets on my nerves and i know it gets on others nerves, but do you think Google made them like that for Honeycomb? so when or if the NS gets Honeycomb they will possibly disable the touch sensitive buttons, because they want buttons on the screen rather than hardware buttons in 3.0. Again just a thought.
I think the decision to not silk-screen them white was an aesthetic one, not one for future functionality. With the blackness of the SAMOLED display, when the buttons are off the face of the phone is essentially one smooth black slab. Very sleek, and very cool.