This isnt such a leap, but I am amazed the product hasnt taken off (I am not even sure if such a thing exists) -
There will be a device which links game playing (playstation etc) to exercise -
There will be some sort of timer, which is a slot-in card for a gaming console. When the time runs out, you cant play the console any more until you add more minutes to your timer
You add minutes to the timer by doing exercise (perhaps the card slots into a device which you could attach to a bike) - this means that time at the gaming console has to be "earned" with real exercise
Another similar alternative is an excercise bike which regulates a power supply - so you have to exercise to actually be powering your playstation, otherwise the power is cut off
I have been meaning to post this for a while, but since today I noticed this, its even closer to reality:
http://news.bbc.co.uk/2/hi/health/6656631.stm