Ian, nicely done and thanks for sharing.
I've been using linux for a few years but I had never pondered remotely triggering my old Nikon D70 over USB … Now that I know it's possible I'll give it a shot from the desktop and, when mine arrives, from the RPi!
When you say it's running of an external battery, do you mean your DSLR, the RPi, or both? Any tips for extended battery life on either one?
Cheers to you and good luck with the photography!
eastpole || http://www.eastpole.ca