Another thing to consider is that if you truely wanted to get more of a workout from Wii Sports (and in the future Wii Fit) you can always modify how you play the games ...
A woman I know, who is a personal trainer, told me that the way she got her generally unmotivated sister to lose weight was to get her to do excercises durring comercial breaks and to replace her candy 'addiction' with grapes, strawberries or other fruits. Not a particularly big change, but over a year her sister lost over 40lbs.
If you were motivated to lose weight by using the Wii you 'could' get (somewhat) light wrist and ankle weights and an aerobic step and incorporate more movement into your games; with Wii Sports tennis you could step up into the swing, and in something like Wii Fit you could place the balance board onto the step.
Small consistent changes are what really makes for long term weight loss.
