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As someone who has installed a couple child safety seats, I can tell you that you have to a exert an extraordinary amount of force to strap the seat down. The child seat, or its base station, when properly installed, keeps the factory seat tightly compressed. I bet the sensor would read that compression as an adult's weight.
As a side note, I've driven the Smart ForTwo, and I agreed with Farhad's assessment of the driving experience entirely. My Toyota Echo is a lot more fun to drive, and gets better gas mileage. It can also carry my modestly sized family of three, which is a real bonus.