When using AcceleratedX (5.1 laptop edition, but it doesn't
look significant), xscreensaver as used by gnome doesn't
turn off the screen (which it does with XFree86, and
AcceleratedX is capable of its own).
What happens if you run 'xscreensaver-command -lock' (or -activate)
from the command line?
It starts the screensaver, of course. This wasn't the problem - the
problem is that the screensaver, after having running for the
specified amount of time, isn't able to switch the screen off to
conserve power. After some time, it goes blank - but not off.
And the reason for this, seems to be that it tries to use an
XFree86-specific extension - XFree86-Misc. This isn't supported with
other X servers. Checking for such a possibility and instead using
standard ways to do it if this extension isn't present would solve the
problem. This shows up in the .xsession-errors:
Xlib: extension "XFree86-Misc" missing on display ":0.0"
xscreensaver-command: no screensaver is running on display :0.0
xscreensaver disabling builtin screensaver
xscreensaver: you can re-enable it with "xset s on"
If the extension isn't there, there isn't a standard way
to do this.