In case it wasn’t already clear, yes, Rainbow Road will be a playable stage in the new Super Smash Bros. game. Below is a glimpse of it on 3DS, courtesy of director Masahiro Sakurai on Miiverse:
Says Sakurai: “It can’t be explained in one picture, but it’s going to be a cruise-type stage.” That’s the version of Rainbow Road that’s in Mario Kart 7. Perhaps the Wii U version of Smash Bros. will feature a different version of the stage.