Can't power down using only keyboard

[Paul Crawford]

With Ubuntu 12.04 (32-bit) I find that I can't shutdown from the login page using the keyboard. The tab key fails to cycle to the power/shutdown menu, and even if I click the top-right icon using the mouse and select shutdown, I can't operate any of the buttons to confirm or cancel the action using the keyboard.
I expect that a computer can be operated fully from the keyboard alone, and in a manner that most users would be familiar with. This is potentially a serious issue for disabled users who lack the motor skills to operate a mouse, etc, with any precision.

$apt-cache policy lightdm
lightdm:
  Installed: 1.2.1-0ubuntu1
  Candidate: 1.2.1-0ubuntu1
  Version table:
 *** 1.2.1-0ubuntu1 0
        500 http://gb.archive.ubuntu.com/ubuntu/ precise/main i386 Packages
        100 /var/lib/dpkg/status