Just thinking off the wall, here.
I would be curious if someone who works with radiation often (rad techs, radiologists, et al.) went through one of those scanners with a one of their radiometers that were clean, and see how much they get with a scan. A lot of the images I have seen are awfully high resolution, which indicates a significant amount of power (radiation,) is being used.
Frequent flyers would be at risk. Health workers are limited to around 5-rem total body exposure beyond background radiation of 0.360-rem a year we are all exposed to. (areas individually can have more or less, depending on area, but full body exposure is what body scans imply.) 5-rem of radiation is equivalent of 3-5 CT scans a year. (source: University of North Carolina Health Care System Radiation Safety Manual, link http://ehs.unc.edu/radiation/manual.shtml and this link here) Even at 0.500-rad, 5 round trips would be enough to give you as much radiation as an average hospital worker is allowed per year.