After some digging I had to realize, that the thinkpad_acpi module is functioning well, the error is in XFCE-s display brightness setter.

This is a duplicate of:
https://bugzilla.xfce.org/show_bug.cgi?id=7541
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=627336

Workaround solution:
http://www.n0nb.us/blog/2012/02/restoring-screen-brightness-step-size-on-xfce4/


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