Unable to shutdown on fresh install of Jaunty running on i686

This server is a fresh install of Jaunty running on i686.

As root, shutdown -r now tells me it's going to reboot. But instead of my ssh session being disconnected I am left logged on.

ps ax reveals the following new processes:

18691 ? Ss 0:00 /bin/sh /etc/init.d/rc 6
18695 ? S 0:00 sh /etc/rc6.d/K25hwclock.sh stop

Running strace -f shutdown -r now gives this final output:
11:49:23 open("/usr/share/locale-langpack/en_GB/LC_MESSAGES/upstart.mo", O_RDONLY) = 3
11:49:23 fstat(3, {st_mode=S_IFREG|0644, st_size=1205, ...}) = 0
11:49:23 mmap(NULL, 1205, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f9c38b0c000
11:49:23 close(3) = 0
11:49:23 open("/usr/share/locale-langpack/en.UTF-8/LC_MESSAGES/upstart.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
11:49:23 open("/usr/share/locale-langpack/en.utf8/LC_MESSAGES/upstart.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
11:49:23 open("/usr/share/locale-langpack/en/LC_MESSAGES/upstart.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
11:49:23 geteuid() = 0
11:49:23 setuid(0) = 0
11:49:23 getuid() = 0
11:49:23 open("/var/run/shutdown.pid", O_RDONLY) = -1 ENOENT (No such file or directory)
11:49:23 clone(
Broadcast message from jamesg@scaramanga
        (/dev/pts/0) at 11:49 ...

The system is going down for reboot NOW!
child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f9c38c42780) = 18690
11:49:23 --- SIGCHLD (Child exited) @ 0 (0) ---
11:49:23 chdir("/") = 0
11:49:23 umask(022) = 022
11:49:23 socket(PF_FILE, SOCK_DGRAM, 0) = 3
11:49:23 getpid() = 18689
11:49:23 bind(3, {sa_family=AF_FILE, path=@"/com/ubuntu/upstart/18689"...}, 28) = 0
11:49:23 setsockopt(3, SOL_SOCKET, SO_PASSCRED, [1], 4) = 0
11:49:23 sendmsg(3, {msg_name(22)={sa_family=AF_FILE, path=@"/com/ubuntu/upstart"...}, msg_iov(1)=[{"upstart\n\0\0\2\0s\0\0\0\10runlevelas\0\0\0\0016S"..., 34}], msg_controllen=0, msg_flags=0}, 0) = 34
11:49:23 unlink("/etc/nologin") = -1 ENOENT (No such file or directory)
11:49:23 unlink("/var/run/shutdown.pid") = -1 ENOENT (No such file or directory)
11:49:23 exit_group(0) = ?

halt, poweroff, and shutdown -h now all fail in the same way.

Prior to the fresh installation this server was running 8.10 which I think had the same problem. At least, during power maintainance I remember one of the boxes having unexplained shutdown problems and we had to switch it off using the physical chassis button.

I am at a loss to explain it!