well, not sure about Wii Fit, but Wii Sports Boxing *could* work as a cardio exercise. If you go all haywire, anyways, you know, throwing punches like crazy, moving around and everything. At least i find it a little more exhausting that way than rope skipping for the same amount of time, well overall exhaustion. Though rope skipping would definitely target different parts of the body. And it does in no way compare to hitting a real bag for half an hour.
But doing that all day could seriously mess up your technique
and when looking at the wii fit homepage. there's really no exercise in Wii Fit that you can't do just as well without the wii and there's a lot of exercises that are imo better suited for conditioning, so it's really not worth spending the bucks on if you just want it for training. Rather invest in some good training equipment.
Or just pay the money you'd have spent on the wii board to some friend/bum/trained monkey and have him tell you "You're doing great", "that's it, you've got excellent balance" or "Pushup!". They'll be happy about the money and if you want to check you're progress, you even have someone around to beat the crap out of