I did get to see what it is the security screeners see on their display for few seconds. Looks like the public reports were right and they really don't get to see much. The display was maybe 8" diagonal with a sort of stylized figure (including hair, so either it's someone else or it's really stylized) with boxes that apparently indicate stuff that was detected. As I understand it, what's going on here is that the real image is shown somewhere else and then some screener elsewhere points out the regions of interest for local handling.
Here's something I've been wondering about: how are those signals transmitted to/from the screening room? Is it wireless or wired? If wireless, what's the security? If wired, do the cables run through an area that's potentially user-accessible. Interestingly, I didn't walk through the magnetometer, which means that the scanner is the sole line of defense for anything you carry on your body. An attacker who could control this network could, it seems to me, suppress warnings from the remote screener and walk through carrying anything he wanted. (They don't really do a complete pat down in many cases.) Another possibility would be to remotely subvert either the screening consoles or the scanner itself. There's sure to be plenty of software in both. Finally—even with a wired network—would be to monitor RF emissions off that network, constituting a privacy threat.
Anyone want to loan me a scanner?