If you tried importing the Japanese version of Mario Kart Wii to Europe or the U.S. in order to get your copy early, you may have received this message from Play-Asia: "We received the first small delivery today for testing and unfortunately we have to inform you, that the game is not compatible with the Datel Freeloader (US and PAL Version)."
However, we've heard from NWF reader Nils that this isn't true, and an imported copy of Mario Kart has worked just fine on his Wii (we're still waiting to hear back on what version he imported). Still, we're sure Play-Asia wouldn't make such things up, since this will only lead to the site losing sales. So, at this point, just be cautious with your imports, and let's hope that Datel's Freeloader isn't outdated already.
We'll update the post if we find or hear more accounts from Mario Kart Wii importers, so be sure to tip us if you've tried it first hand.