I think Nintendo (or maybe more likely Nintendo and their third party partners) have killed a lot of the enthusiasm for the Wii by letting it go through all these droughts and not having a big guaranteed seller last Christmas.
When you've lost that enthusiasm among consumers for your platform, I think it can be very difficult to get it back. That Wii Sports Resort could not create a sustained boost in hardware probably means Wii Fit Plus is in the same boat.
Which basically leaves it all on NSMB Wii to do all the lifting this holiday season.
Nintendo really has to get over this idea that they can go on vacation (basically) for long stretches of time and then just show up with one or two big releases and think everything just goes back to normal. Doesn't work that way.