Comment 1 for bug 97251

After a little fiddling, I've discovered that gnome-screensaver is turning the screen off. This is despite the fact that totem is running in full screen, and despite the fact that the screensaver has been disabled ("Regard the computer as idle after: is set to "2 hours" , and "Activate screensaver when computer is idle" is unchecked.

Also, in gnome-power-manager, "Put display to sleep when inactive for:" is set to never.

I've determined this by running a video for 20 minutes without touching the mouse and noticing that the monitor goes to sleep kicks in.
I then ran "killall gnome-screensaver" and ran the video again. This time the monitor did not turn off.