Mario Kart: Double Dash!! (Note that the game DOES have two exclamation marks) will be coming with a demo disk if you pre-order it.

While the Zelda bonus disk was cool, this was probably done to ensure that those who hated the cell-shaded look would buy the game. Mario Kart really has no controversy of that scale and thus Nintendo is just offering a small treat for those who pre-order.

EDIT: It's very easy to suck at Mario Kart if one person you play with ALWAYS gets the item they need. I hope the character-specific items will reduce the randomness a bit.