I finally got the XP part of the mac to stop treating the projector as an extended desktop. Just in time for the arrival of the ebeam interactive whiteboard. It now works as I had intended all along.
If only its XP personality didn't take quite so long to boot up and log on to the network........
(Added 1/3/07 - Instructions for mirroring desktop on projector to be found in new 'Mac PC Book Win Pro XP' section in 'My Work' on the right - I should have done this before, but it was a bit like Peter Pan flying - it worked as long as I didn't question it!)