The japanese page for the upcoming Mario Sports Mix for Wii reveals that Square Enix will be handling development duties.
According to Siliconera the RPG giant will be working on the next sports game for Wii. Square Enix has worked on Mario games before, the first being the sublime SNES epic, Super Mario RPG, and the second, a more relevant Mario Hoops 3-on-3.
So with the experience learnt from 3-on-3, the team will be pushing the concept onto the Wii with various team based sports for the Mushroom Kingdom folk to enjoy.