I got asked to 'volunteer' for this when I was going through T4 last week. From what I understand this isn't an x-ray technology, the terminology I've heard being used is 'millimeter wave' which is more like radar, in fact. I believe the intention behind it is that it just penetrates the clothing layers, allowing them to see if anything suspicious is strapped to somebody.

When you're finished getting scanned they allow you to see the imagery that they have collected, I had a quick look and it seems quite impressive. It doesn't look like an x-ray - you can't see any bones! - but I could clearly see my watch on my hand, which was the only metal item I was wearing. And of course the big advantage is being able to skip the whole queue in the security line!

I've heard that this is only a very early version of where they actually want to take this technology. Apparently it can be adapted to be placed inside the walls of a corridor, so people could be 'scanned' as they walk through a particular area of the airport, for example. This could be very impressive - if this was the case, people wouldn't even have to stop walking, but imagery would be available from multiple directions to see if they had anything suspicious. Could be interesting...