I think Nintendo will be coming up with a new handheld soon, separate from the DS. I'm thinking they'll be using an accelerometer/touch screen similar to the iPhone/iPod touch.
Despite what their next handheld will be, since Nintendo is probably decently far in their plans on it, I wouldn't expect another Mario Kart on the DS. The team that works on the Mario Kart games will probably move onto the new Nintendo handheld once they are done with Mario Kart Wii.