Here's a good, measured run-down
of some of the possible safety concerns about those backscatter x-rays. The issue isn't that there's clear evidence they're unsafe but rather than this particular technology operates somewhat differently and may be used somewhat differently than the x-rays used in the studies that supposedly prove their safety.
Interestingly, there's another technology, millimeter-wave scanners, that doesn't use x-rays at all and seems not to have any health concerns tied to it at all. They're in use in a few airports but not many. Ironically, for those worried about junk-exposure, they seem to produce slightly more revealing images than the backscatter junk detectors.