Command-Line, 11.10, Shutdown, Dialog – Command to Open Shutdown Dialog

you can try experimenting with different kernel options. try one at a time and see if it has any effect

like for example:

acpi = force or acpi = off

to try one out , when grub(with the selection screen of ubuntu-linux kernels, pops up after you have booted your computer,

press the "e" button to edit your current configuration.
to the line that has the words single quiet, add one of the proposed otions above.

this setting is not permanent over reboot so there is no risk of wrecking anything. if this works, and you want to make it permanent its time for a follow up question :)

Try:

gnome-session-quit

For more options:

gnome-session-quit --help

According to the manual, the following options are supported:

   --logout
          Prompt the user to confirm logout. This is the default behavior.

   --power-off
          Prompt the user to confirm system power off.

   --reboot
          Prompt the user to confirm system reboot.

   --force
          Ignore any inhibitors.

   --no-prompt
          End the session without user interaction. This only  works  with
          --logout.

Or looking at dbus-monitor, this is the command:

dbus-send --print-reply --dest="org.gnome.Shell" /org/gnome/SessionManager/EndSessionDialog org.gnome.SessionManager.EndSessionDialog.Open uint32:2 uint32:0 uint32:60 array:objpath:/org/gnome/SessionManager/EndSessionDialog