SLF, if you put a slab of Semtex in your camera, the extremely tiny spark when you take a picture may set it off - unless the camera is entirely mechanical, rare these days. The powers-that-be assume that people who have got Semtex airside are going to try and put it on a flight that they aren't on themselves; therefore this test won't stop a suicide bomber. It just mean he gets blown up in a capacious airport rather than the more vulnerable environment of an aircraft.
Aah, I hear you say, but I wasn't talking about a camera, I was talking about a Psion organiser, which has a membrane keyboard, which doesn't spark or even generate high external electric fields! Well, you can't expect airport security to have in-depth knowledge... They are just given a simple set of rules to follow mindlessly. Namely, ask the pax to switch on any electrical items.
The threads drifted a bit from what I put up originally, but so what, it's still interesting, indeed becoming more so. Thanks everyone for your comments.