400k 1st day, 1 million 1st week, 2.5 million first month
Without direct competition of SSBB and Wii Fit it is by far the biggest title on the Wii to justify my numbers. Atleast 7 million Wii owners by the end of the year to be hungry for a new game, so a good chance my estimate is too low.
Edit: lifetime 6.5 million