Loading the power manager causes xbacklight to work incorrectly, providing only 2 brightness settings, minimal backlight and max, regardless of how little you increase/decrease the brightness by. Unloading the power manager lets xbacklight work normally. In addition, using the brightness hotkeys causes the backlight to flash wildly and also locks up the computer, requiring a hard reset if a shutdown is not initiated fast enough. The brightness plugin that comes with the power manager works fine both with and without the power manager loaded. This is all on a Samsung N150, with the samsung-laptop kernel module force loaded for backlight support. I don't know what else to provide so ask away.
There is now an option (since 1.3.1) to disable the handling of brightness keys by xfce4-power-manager in its settings dialog, so for those that have problems with it can disable it easily.