I hate to admit it but, I think it is a good idea.
It makes me think of the TQM idea that if you pay attention to an indicator it will move. It may not cause you to make a different selection in McDonald's but it may cause you to eat lighter or exercise a little more later in the day.
I think they should add a siren and flashing lights when your order goes over 1,500 calories and maybe a digital scale mat to record and flash your weight on a Jumbotron when you step up to pay the cashier.