Turns out that both linux mint and ubuntu 11.10 don’t have xscreensaver anymore as default. This seems fine in Ubuntu as there is a simple fade to black when needed. Unfortunately in Linux Mint it has a butt ugly colour flashing reminiscent of monitor test patterns.
To fix this you need to remove gnome-screensaver and install xscreensaver. The details can be found in the linux mint forums, here.