Just to give a little extra info on how potentially big this is, Modern Warfare 2 360 version has currently sold 6.73 million copies in 4 weeks. New Super Mario Bros Wii is looking at about 4.9-5.2 million after 4 weeks on the market. PS3 version of Modern Warfare 2 has sold 4.43 million in 4 weeks. So New Super Mario Bros wii, despite maybe not having the opener of the same magnitude, is definitely catching up in the weeks after.
Actually NSMB Wii will more than likely pass up the PS3 version of Modern Warfare 2 with this upcoming week's sales even with the 100k bump thanks the Japanese release of the game. And to really put something in perspective that is kinda scary, NSMB Wii has an outside shot of having more copies sold than the 360 version of MW2. Considering most expect it to sell another 1.5-2.5 million copies in the last 4 weeks, or middleground of 2 million, New Super Mario Bros Wii still has that shot (would put 360 version at 8.23m-9.23 million range).
So indeed New Super Mario Bros Wii is looking to be as big as it has been touted. Despite the possibility of my Japanese numbers potentially being a little to high and more likely being around the minimum prediction rather than the middle ground, it does seem that Mario Wii is set to be quite a record breaker itself as one of the fastest selling single games ever.