Thanks LB, also others for the replies by email.
Alas the answers shot my theory down in flames - which in a way is a good thing because it's spurred me on to investigate deeper to see if I could see what was going on. Which I think I did.
I have to apologise because I was slow to blame the einSDein design.
Every board had tested fine before being sent out. I assumed that there wasn't a problem, and didn't think that an issue could have been masked by my particular set-up. When I tested the boards I assumed that every card would behave roughly like the one I was using, a relatively modern 16gb sandisk. Since then I've tried a 32mb (yes, megabyte) one, some 64mb and a couple of 2 & 4gb cards. All seemed to check out - perhaps because my test set only includes cards that work with all my various devices. All the ones that didn't work were thrown out ages ago because I assumed they were old and defective - not that there could have been an issue with the thing they were inserted into. Hubris!
Anyhow to cut a long story short, err that is once it's already been a long story and you've read it anyway, I've managed to reproduce something like what you guys are seeing on one of my boards. A bit more extreme but nonetheless it fits roughly with the pattern - unreliable reboots, works for a while then doesn't respond to reboots, white light on the whole time at boot. The other boards I have all work, so I have to assume it's down to 'tolerances' /waves hands vaguely/
I've spent the day with an in-circuit debugger and found what I think is the issue. A test board is off to someone tomorrow and I hope to have great news shortly after. The sad irony is that I'd anticipated something like this issue and speculatively placed pads on the board for an optional component in case it cropped up. A small mercy I guess
Anyhow, I was nearly right and I'm one pull-up short of a full set.
So thanks for your patience everyone - a day or two more and we'll be a bit closer to an answer! If I've cracked it then it's a very simple mod to the board and we're off again.
C
) to have the keyboard and associated weirdness looked at. 
