With some help from d-u I finally got my Canon S1 IS working under Debian.
I was concerned because the usual messages that I get when plugging in a USB device about SCSI emulation were missing. However I saw that some people had it working, and that it was in the list of supported cameras for gphoto2. The key was to avoid autodetection and to tell gphoto2 to use PTP mode for my camera. I also had to add myself to the camera group. In the end this was the command that lists all of the images on the device
gphoto2 –camera "Canon Powershot S1 IS (PTP mode)" –port=usb\: -L
So that's another thing that I don't need XP to do, and the list of things that it do need it for is tiny now. Maybe one day I'll get round to removing it completely, after all I haven't booted in to it for a long while.