The Nintendo 3DS has 3D and a gyro sensor. That doesn't the 3D gaming and motion sensing gaming go together on the 3DS like peanut butter and jelly.
The 3DS has a sweet spot for viewing 3D. Tilt the console to the side, and the 3D effect vanishes. Gyro sensing is based on tilt controls!
In a recent issue of Japanese magazine Nintendo Dream, Mario creator Shigeru Miyamoto pointed out that it would be hard to have motion controls in a 3DS race game.
While the 3DS is capable of detecting when it is being tilted and still showing 3D, this might be difficult for a racing game as the viewing range could be more significant for real time racing games when compared to other types of games.
Perhaps Nintendo will figure something out — I can't imagine them bringing Mario Kart to the 3DS and not taking advantage of the system.