Thank you all for your very helpful comments.
Status at the moment is: I have a simple game of life working (where each peer only handles one GOL square - obviously this makes network traffic massive!) but need to make the Organizer (which tells peers where they are and who their neighbours are) more reliable. The Organizer is clientserver based at the moment - it is easier to have one peer marshalling all the others into position before they all start talking to each other - but it's nicely subclassed so a truly self-organizing space will be quite easy to write. (Essentially, when a newbie peer arrives,each peer "points" it to the next peer, until the newbie gets to the frontier and finds a free position; all peers know their universe size and don't let anyone get beyond it.)
dave hj~