Pure speculation on my part (i.e. not based on anything but my opinion), but I'd say Wii Fit will move roughly what Super Mario Galaxy did in week one. However, I think it will decline incredibly slowly (if supply is there). Galaxy went from 286k-45k in the prelim data, and 255k-75k, 250-80k by Famitsu/MC and VGC in weeks one and two.
Wii Fit I can see doing like...200k...160k....130k...100k....70k...55k....
In contrast to Galaxy which will end up more like: 250-80-40-30-30-60-60-75-50-20-20-10-10
People are difficult to govern because they have too much knowledge.
When there are more laws, there are more criminals.
- Lao Tzu