nope - I've tried that before as well.
as a curiosity - I just tried it running 3.2.29"
# xbacklight -set 100
No outputs have backlight property
wtf? I _know_ I have tried it before with no luck - probably under archlinux as well (it's fairly recently - as from 14.0 - that I have installed slackware on absolutely all my machines - earlier I just used it on servers)
Actually, its not only an X problem - it also happens as it boots ... but then the nouveau driver is loaded fairly early in the boot process.