It's already been announced that Smash Bros Brawl will use the Gamecube controller.
Source: Click meOriginally Posted by Joystiq
Interesting bit:Hope that clears everything up. Hopefully, they will keep selling Gamecube controllers for a while (so the people who don't own a Gamecube can still buy the controllers needed). Either that or ebay...The game will feature online multiplayer. Single-player mode will see some changes, but Sakurai has declined to elaborate. Miyamoto did say (via translator) that we can expect "a very robust" single-player mode.