Sometime early next year, I've heard. And there's other violent games on Wii too, you just have to look.
The developer specifically said they wanted to make a type of game for Wii that isn't widely available for the console. Technically, Nintendo doesn't have to 'okay' it. They don't really have anything to do with it. Personally I think we need more of this type of game on Wii and I'll be buying it day one!
And yes, black and white with red blood. Like Sin City.