Comment 2 for bug 58505

The suggestion (thanks to gnomefreak) was:

 sudo apt-get install xserver-xorg-core xorg xserver-xorg-video-all

Of these, iirc xserver-xorg-core and xserver-xorg-video-all were already satisfied. What had not been pulled in was:

  'xorg' and its dependency 'xserver-xorg'

There were the missing packages after the upgrade. Unfortunately I can't tell you want what installed before that; I didn't keep a log of the upgrade. Perhaps I didn't have xorg installed previously and it wasn't required for my config in dapper?