Super- it's there and when I add the motion_mask setting and quick-reload it definitely updates /etc/motion/motion.conf.
And I can confirm that the masked-out areas are no longer firing motion events which is certainly an improvement in my case.
The odd thing, though, and still to be confirmed, is that I didn't seem to have any video files. I had snapshots being generated, and definitely motion events because my gtalk is being notified, but there were no video files in /home/cameras/....
Will check that one out. Maybe I mucked up the config file somehow.
I've never set up more than one camera but I'm close - he camera is there, the cat-6 cable is there, I just need to hook it all up. Now, when I do, will there be a new motion-wrapper for the new camera? Presumably there is one wrapper "device" for each camera? Otherwise things will go South as I can't use the same mask for each camera!