Comment 6 for bug 295560

my conclusion is that gnome power manager can:
notify change events to the user (brightness bar) (ok here)
dim the screen after a period of inactivity, if user wishes so. (and set it back the previous level) (probably ok too, when the xrandr issue is fixed)

and must not:
1: use xrandr
2: set brightness upon login/battery/ac events

A couple of new gconf settings could give the user enough control to impose this behaviour.
eg brightness_method = auto | hal | xrandr | wathever (fixes 1)
brightness_on_power_change, a boolean (fixes 2) (should also disable "set brightness to" slider in preferences)