UPDATE : US RELEASE CONFIRMED! One Piece announced for WiiU (and PS Vita/PS3)

[Kresnik]

orniletter said:
Yeah, this is a game you want to play with friends.

3DS it is....and maybe WiiU.

https://www.facebook.com/media/set/?set=a.678569178870836.1073741868.233213603406398&type=1&stream_ref=10

"* Nintendo 3DSTM and PlayStation®Vita will feature local multiplayer gameplay for up to four players. Wii UTM and PlayStation®3 will feature split screen multiplayer gameplay for up to two players."

I hope your friends all have 3DS' :P