I didn't see us speculating on this (maybe I missed it), but did spot it in the Go.Nintendo comments: Andy is the one sticker we've seen in this update with a black background. And I went back and looked at the stickers update--and, yep, Andy's still the only one with a black background.
Some were considering the possibility that it might mean Andy was playable, but that doesn't seem true; Ike, Donkey Kong, and Kirby have various stickers with white backgrounds.
So what do you think it means? I'm sure someone will come up with the answer if we brainstorm.