New Super Mario Bros. Mii is a simple tech demo showing off the gameplay on the Wii U controller itslef. Nothing too impressive here, as the tech demo is simply New Super Mario Bros. Wii with a few higher resolution textures, the ability to play as your Mii, and five new stages to play through. The game is not set for an official release, but Miyamoto has stated that the new Mario Wii U game will be based off this tech demo.
http://www.youtube.com/watch?v=jk64r7E2wUM