Thanks, Richard and Peter, that helped a lot.
I'd read the intro and the FAQ but not the wiki. I shall probe the last but not until (apply industrial strength matchsticks) the morning in the old fashioned and comfortably un-pedantic sense of the sun coming up.
My primary interest just at the moment is having a low power device server that I can program to take various USB devices and also do provide some very basic network services that need to be on all the time if they're going to be any use at all. However naturally if the technology has wider implications I'd be interested to know where it's going, just out of curiosity. The fast booting sounds attractive.
The use of the main processor kills the thought of motherboards gaining a second, low power purpose in their powered down mode. I suppose one could use wake on lan, use the processor for a short time and then shut down again however that isn't ideal. I've looked at some purpose built embedded devices that run on linux and I'll probably use one of those. There are quite a lot to choose from!
Once again, thanks for your help.
Best Wishes, Max