Technically our good friend Mario has been available on the Wii since launch via the Virtual Console. But as far as most people are concerned the first new Mario game for the Wii will be Super Mario Galaxy. Which isn't quite true. Mario's on the Wii now in the form of Super Paper Mario (links: GameSpot, GameSpy, GameFly).
While Galaxy is going to be a jaw dropping 3D glitz explosion, Paper Mario is low tech. That's not to say it looks bad - its flatness is quite charming. For those of you that played Paper Mario: The Thousand Year Door on the GameCube, you know exactly what I'm talking about. But instead of being an RPG with some platformer elements, Super Paper Mario is a platformer with some RPG elements. In other words, it should appeal to more people. I found The Thousand Year Door to be a high quality RPG, but RPGs aren't for everyone.
And given that it's primarily 2D, Super Paper Mario may hit that sweet spot that New Super Mario Bros did for the DS: simple and classic. I haven't had a go at Mario's new Wii adventure, so I can't say for sure. But consider it at the top of my queue.