Before you ask: yes, it's a Gamecube controller. Of course it is. What else would it be?
Not content to play Smash Bros. with the 3DS, だいこん went ahead and modded their system so they could play using a controller. Honestly, when people are destroying their circlepads while playing Smash, can you blame them? It's probably such an improvement that all the cables become worth it. I say this as someone that's been playing the full game for the last week, and boy, what I would give to be able to play it like this!
I'll have something more in-depth about how the game controls on the 3DS sometime later this week. For now, we can all jealously look this mod. Sigh.