I have tracked down the problem. It isn't xorg that's at fault but udev, which was updated in the same sysup operation.
udev used to create a device (or perhaps a link - I don't know which) called /dev/input/mice, and this is what my xorg.conf file specifies as the mouse device. Now it isn't created anymore. When I created a link of that name by hand to /dev/mouse0 and restarted X, the mouse worked as before.
Obviously that isn't a proper solution, it's a bodge. So what should I do? Should I change xorg.conf to point directly at /dev/mouse0 or is there a way to make udev create /dev/input/mice at boot again?
Hazel Russman <hazel_russman(a)yahoo.co.uk>