There is a site named Comgnet that ranks preorders for games in Japan and they update on a daily basis. They have a system with points and don´t ask me how it works but apparently they are trustworthy and a good tracker for preorder numbers. The big news is that they are tracking New Super Mario Bros Wii way higher in preorders than what New Super Mario Bros for DS did in the same timespan.
Today:
NSMB Wii with 465 pt (+43) (NSMB DS was at 289 pt, +21)
As you can see NSMBW is much higher and gaining much faster on a daily basis than the DS game. It´s not certain that the game is going to debut higher then the DS one but this is still very good news. This points to a huge hit for Nintendo when the game is released in a little over a week. I will predict right now that it will become the biggest debutselling game for Wii behind only Brawl and possibly even beating that one a scoring the best first week ever for a Wiigame in Japan.
My prediction for the first week: 750-950 k. It´s going to be massive folks!
Edit: New no guts no glory prediction: 900-1100 k. I think it will debut higher than New Super Mario Bros (did 856k, Famitsu)
If you want to have a look at Comgnet I found this site on "Nipponsalesage". There are links there to some other interesting sites as well.
Chart archive: website