I'm almost certain that an NT based OS won't start a screen saver once another is already running
Well, that being the case its not quite as catastrophic as I thought it was. But even so, one type of behaviour in DX and something different under GL is confusing at best. They should both be the same and I can't think of any logical reason why they aren't. I haven't got a clue what happens under Linux and OSX. :o/
At this point in time, I don't much care. But I'm sure its something that needs looking at.