Play-Asia's just put the following message to those with pre-orders for the Japanese version of Mario Kart Wii, according to GoNintendo:
This is an important e-mail update regarding your pending preorder Mario Kart Wii (w/ Wii Handle) (Nintendo™ Wii Japan, NTSC J)
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).
In case that this affects your preorder, please contact our customer service as soon as possible to amend/cancel your order, as otherwise we will ship your order within the next 48 hours.
If you own a Japanese Wii and you do not want to change your orders, you do not have to reply to this email.
Once again thanks for shopping with us and sorry for the inconvenience caused.
That's a shame. Or not. Depends whether or not you're Nintendo Europe, really.