Comment 7 for bug 202800

"Strange is the fact that occasionally it works perfectly, and sometimes it fails completely."

As it turns out, Ubuntu has a strange way of defining idleness. Everyone else uses human input (mouse, keyboard, etc). Ubuntu is using cpu as well, and there is no way to unselect that part of it. So if your cpu is busy enough, you are not idle and the screen will not blank.

See here for more info, there is talk that it is fixed in Lucid.

https://bugs.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/289322