It's definitely worrisome. Radiologists and radiology technicians go through quite a bit of training to learn how to avoid overexposing someone, yet things still happen. A lazy CT technician keeps pushing a button on a CT scanner and causes a ring of radiation injury to a kid. No offense to TSA people but I don't have all that much confidence in them thinking first about my health.

Would I subject myself to the radiation if they told me they could guarantee my safety? Sure; it's a very worthwhile risk. But just as I refuse chest X-rays when the doctors can't tell me what they expect to find (or as another poster mentioned, would choose an ultrasound or MRI over a CT if they were equally effective), I don't see why I should go through a backscatter device rather than a metal detector or pat-down if they are apparently just as effective.