Comment 30 for bug 66257

Right, I just had this problem on latest Feisty. With gnome-power-manager set to never deactivate the monitor and gnome-screensaver set to never enable the screensaver, the screen blanked out after 20 minutes.

xset q:
DPMS (Energy Star):
  Standby: 0 Suspend: 0 Off: 0
  DPMS is Enabled
  Monitor is On

Running "xset -dpms" seems to make it work, and xset q now shows:
DPMS (Energy Star):
  Standby: 0 Suspend: 0 Off: 0
  DPMS is Disabled

I suppose a workaround is adding xset -dpms to your startup programs, in theory. Anyway, an upstream bug in gnome-power-manager linked above seems to deal with this exactly and even has a new patch as of a couple weeks ago, so I suppose I'll assign it to gnome-power-manager and set the upstream watch.