Sneaky Nintendo—at the end of a video advertising their upcoming Smash Bros. tournament at E3 in two weeks, the house of all things Mario just announced a GameCube controller adapter for the Wii U.
It's unclear how this will be sold, if it's sold at all. Nintendo says the Wii U version of Smash Bros. will be playable with GameCube controllers, so presumably we'll be able to get our hands on this thing in some way or another:
We're asking Nintendo for more details. In the meantime, you can watch the full Smash Bros. Invitational trailer below.