The zonbu PC that managed the backup disks target of duplicity, which was running Debian stable has died.
I tried and restore the contents of its system (to have a look on the config files I had setup there) on a Debian testing system, using the same duplicity commandline (more or less).
However, I couldn't do that, since apparently, duplicity in Debian stable (0.4.11) and testing (0.6.06) don't seem to be exactly compatible.
I managed to restore anyway by reinstalling duplicity 0.4.11 in a custom prefix setup, which worked fine. The command-line then goes something like this (excerpt from the tarball's README) :
python setup.py install --prefix=/usr/local PYTHONPATH='/usr/local/lib/python2.x/site-packages/' /usr/local/bin/duplicity -V
I've traced this problem in Debian BTS (#572102), as I'm afraid of the consequences when people will try and restore on the next stable distro backups made with the previous stable...
You've been warned anyway ;)