The American Council on Exercise has just released a small study suggesting that the Wii Sports games are better than sitting on your butt but not as good as doing the sports themselves.
In descending order of caloric burn, Wii's version of boxing burned an average of 7.2 calories a minute, tennis burned 5.3 calories a minute, baseball about 4.5, bowling about 3.9, and golf about 3.1. (That means 30-minute workouts burn 216, 159, 135, 117, and 93 calories, respectively.) That's less than the real-life activity in all cases; real bowling actually burns about twice as much as the Wii version. And only Wii's version of boxing counted as strenuous enough to meet the ACE's guidelines for endurance-building exercise.