Comment 4 for bug 665195

Ah, sorry for misreading. The thing is that plymouthd is supposed to be run *twice* on the system, once at startup and once at shutdown. I assumed that what you were seeing was a plymouthd process running at shutdown like it's supposed to; the open /var/log/boot.log (and the --mode=boot argument to plymouthd) show that this is actually the process that was started at boot time.

So it sounds like the problem here is actually the combination of

 * /etc/X11/default-display-manager is set to /usr/bin/xdm

and

 * gdm is also installed

Because the gdm job starts up, /etc/init/plymouth-stop.conf is triggered with JOB=gdm, so it exits without killing plymouthd. But then the gdm job looks at /etc/X11/default-display-manager and decides not to start, dropping the ball - nothing stops plymouthd in this case.

So the bug is in the gdm upstart job; reassigning to gdm.