I found this excellent post from a few months back, as well as the back-and-forth in the comments, to be very interesting. The blogger is a biochemist (like myself) and talks about what he knows about radiation and cancer mechanisms (all of which is accurate based on my own knowledge).
When scientists, such as Professor Sedat at UCSF or little-ol-me, complain that there has been no peer-reviewed studies of these machines, this is why. Just look at the comments section on the blog post - there is a lot to consider, weigh, and evaluate - just from a technical standpoint. None of this has been done, and yet the line fed to the public is that these are safe.
BTW - if you want to check out how absurd the propaganda is on this, check out the TSA's "More Information" page on the scanners. The "Media Coverage" section has only two links, both to CBS News, and one of them is this tripe from when the controversy gained steam last fall. Do you think that CBS journalists get their paycheck directly from DHS, or is the money laundered first?