tags 676854 + fixed patch pending thanks I confirm that the patch referred to in the previous link (https://bugzilla.xfce.org/show_bug.cgi?id=8840) fixes the problem (I'm attaching it now with minor modifications so that the format looks more similar to the patches already included in the xfpm debian source).
I applied against the wheezy 'debianized' source, then compiled and built the xfpm packages using 'dpkg-buildpackage', which I later installed on my system. Now brightness auto-adjust handled by xfce power manager works as expected in all the tests I've done. Please include this fix in the next version of the packages. Why has this not been upstreamed yet?
06_fix_dim_on_inactivity.patch
Description: Binary data
