Comment 18 for bug 478274

Bingo ! Thank you very much Chris ! You were right: it was a local configuration issue. But I'd say consolekit seems overly sensitive and I'm not sure its behavior is really legitimate.
Here is what /usr/sbin/console-kit-daemon --debug tells me (in /var/log/daemon.log):
Nov 15 10:50:07 xyz console-kit-daemon[940]: WARNING: Unable to load seats from file /etc/ConsoleKit/seats.d/.svn: Not a regular file
So, indeed, I started (shortly before the upgrade to karmic but I didn't notice that bug in jaunty) to have my /etc managed by subversion, and this caused console-kit to fall apart. By moving the .svn directory away, I confirm that the system now behaves as expected (gdm drums that I will promptly shut off, login melody that I will promptly turn off too, mount usb stick, system shutdown, etc.).
However, I am not sure that having console-kit-daemon half-fail (and "half" only) because it sees some directory where it expects only regular files, is a perfectly nice behavior. I'd rather it fails completely or simply ignores non-regular files.
Anyway, I should have noticed that problem before, as it has always been reported to daemon.log but I did not notice it (looking for the wrong log files and/or log message prefixes).
Thank you very much for your very valuable help !