Nintendo has a tradition of selling accesories with games, started with Duck Hunt, passing through Star Fox 64 and recently Link's Crossbow Training. So it's highly likely they will sell a 4GB SD card with a game, but since the accesory isn't as appealing as the new wiimote that comes with wii play the game has to be a guaranteed system seller.
Animal Crossing would be a good game to bundle with an SD card, you could save items in it and transfer them to a friend through it if there someone doesn't have internet! But I'm not sure if Animal Crossing is a system seller, maybe just in Japan... Of course the cost of the game would be huge because AC must cost considerable money to develop since it has many elements.
A low cost game I believe will sell greatly (but isn't officialy announces yet as far as I know) is Nintendogs Wii. Seems like a very low cost game to develop giving all the profit it could bring from millions of copies sold, bundled with an SD card would make sales go through the roof. I consider it Nintendo's best option for bundling a SD card with a game. But maybe I'm just crazy...
