I'm using v2.6.31-rc1-228-g2bfdd79
and Debian's xserver-xorg-video-intel 2:188.8.131.521-2
when using kms xbrightness doesn't find a brightness property for any display (LVDS connected only). Without kms everything works fine. Chipset is:
(II) intel(0): Integrated Graphics Chipset: Intel(R) Mobile Intel® GM45 Express Chipset
It's a known issue for KMS.
*** This bug has been marked as a duplicate of bug 20963 ***