Super Smash Bros. for Nintendo 3DS will reportedly not support the Circle Pad Pro attachment for the 3DS or 3DS XL.
Last month, a 3DS upgrade was announced with new features, one of them being a built-in C-stick, said to function the same way as the Circle Pad Pro. Despite this, the official page for the CPP in Japan specifically rules out Super Smash Bros. for Nintendo 3DS support.
In past Smash Bros. games, the C-stick could be used as a quicker way to execute powerful smash attacks. For the time being, it looks like players on current hardware will have to stick to the somewhat more involved method of tilting the control stick while pressing the attack button.