As of May 20th, the Nintendo DS/DSi and the Nintendo Wii will no longer have the wi-fi connection service. That means games such as Mario Kart Wii will have no online multiplayer functionality. The 3DS and Wii-U are unaffected, and you’ll still be able to access the Shop on your DS and Wii.
Most games on the above mentioned consoles were single player or had poor multiplayer functionality, but Mario Kart fans seem to hang in there for a long, long time. It will be sad to lose access to such a great game… But Mario Kart 8 is just around the corner, so there’s that to consider.