Attention!
NSMB Wii will NOT update your wii to 4.2!
I tried it myself. I bought a CD, played it when my wii was 4.0, it told me to make a system update. It was like dropping a 100-ton brick on my anticipation, but I wasn't going to let some silly update prevent me from playing my new game, so I made a backup of my wii using BootMii, and was prepared to use Brawl to reinstall the HBC (or use Gecko through brawl, either way works), and updated.
However, I didn't go to 4.2. I checked the Wii Settings and I was at 4.1! NSMB Wii will force you to update, that is true. But not by going through WiFi or internet! The Update is inside the CD itself. If you get a message to update your Wii, update it through the Disc Channel instead of going to system update. You will be able to play NSMBW with 4.1, and I know I am able to (currently at world 8-7 after using warp cannons lol). I think they got the message that 4.2 sucks **** after it bricked so many innocent Wiis, either that or NSMBW was released too soon after 4.2.
I'm still perfectly able to use music and texture hacks alongside with BBrawl, and also enjoy NSMBW at the same time.