Ricky, just because you think it's stupid doesn't mean you'll be forced to play it.
I see it as a huge importance that Nintendo have at least one GTA title on the Wii. GTA is a huge seller, and it was sort of one of the reasons for the GameCube didn't do so well... Including GTA in the Wii's line-up will really shake off Nintendo's kiddie image once and for all. And yes, that image still exists.
The answer is simple: Give. Mario. Guns.
Seriously though, I can understand where this whole 'kiddie' image is coming from. As games grow even more advanced and have more attention to detail, they're going to sway one way or the other. Look at the first Super Mario Bros. You can't exactly call it childish, nor can you call it 'mature'. But looking at more modern Mario games, say, Super Mario Sunshine for instance, you can clearly tell it's swayed more towards a younger audience. It's fun to play, but it's still more on the 'childish' side of things.