It appears that those who were worried about Japanese copies of Mario Kart Wii not loading on US Wiis through Freeloader may have been breaking a sweat for nothing. Import retailer NCS tested the game on v3.1U firmware and found it to run just fine. Yes, this information directly conflicts with an earlier report from Play Asia, but there's a good chance that Play Asia tested the game with v3.0U firmware which NCS confirmed will not play the game.
In other words, all hope is not lost. Still, go forth at your own risk, there may be bananas around the bend.
NCS News Update [NCS]