Nintendo revealed on Twitter that Super Smash Bros. for Wii U will be playable with Nintendo GameCube controllers via a new adapter.

Get excited! The #WiiU version of #SmashBros. will be playable with GameCube controllers. pic.twitter.com/nmyj9QMsuU

— Nintendo of America (@NintendoAmerica) May 29, 2014

Super Smash Bros. for Wii U is currently set for a winter 2014 launch, it will be playable at Best Buy during the week of E3.  Super Smash Bros. for Nintendo 3DS will be released summer 2014.

Source: Nintendo of America