Wow - what a great idea! Just read about it in Readers Digest.
Basically, the Entertrainer is a heart rate monitor that comes with a wireless transmitter to your television. It monitors your heart rate, and if it dips below the level you want to exercise at (fat burn, cardio, or aerobic, or you can set your desired heart rate range manually), the volume on your TV goes down. If you're working too hard, the volume goes up, telling you to slow down. It works as a universal remote, essentially, with the trigger for the volume control as your heart rate.
Want to watch American Idol or some other favorite show? You're not going to be able to hear it unless you keep moving. This would also be good for physical-type video games (like Dance Dance Revolution).
I think this would be a GREAT motivator for those of us who do our cardio while watching a favorite program. It also works with almost all stereos that can accept an infrared remote control signal too, so if you listen to music during your workout, it could be used for that too. Also, for fitness videos, etc, it would be a great way to monitor your heartrate to make sure you're in the range you want to hit.
Thought I'd share - I thought it was a really neat idea. And not too terribly expensive, either (140 dollars - but it includes a heart rate monitor and the transmitter).
http://www.theentertrainer.com/