Comment 7 for bug 415589

Yet another update,
Not starting gnome-power-manager at all after the login, still the display will turn off after specified timeout, just not completely, i.e. the backlight is active, but this is another bug, which is already reported.
So the devkit-power-daemon probably works ok, it's just the fact that auto starting g-p-m somehow prevents it from working.

P.S. I feel like I talk to myself here :)