Comment 10 for bug 385633

some other bugs with similar symptoms:
bug #231397
bug #193192

in addition to the reporter's comment about bug #203513

from bug #231397 (this may or may not help):
Tony Houghton (h-realh: 0) wrote on 2008-10-14: (permalink)

I've discovered that this behaviour is deliberate and
gnome-power-manager (gpm) relies on gnome-screensaver (gss) to
implement the blanking and DPMS instead of X. It behaves correctly
when gnome-screensaver is running.

It would have been nice to be able to use gpm without gss, because for
most people there's no point in a screensaver that does anything
except save power. However, gss does provide a way of suspending
blanking for video players etc:- gnome-screensaver-command --poke -
which is better than temporarily changing the X settings, so it's got
its uses after all.

Although I think it might be preferable for gpm to handle DPMS for gss
instead of the other way round, I don't mind if this bug is closed
without anything being changed.